WO2018169289A9 - Optical fiber laser generation apparatus - Google Patents

Abstract

The present invention relates to an optical fiber laser generation apparatus and, in particular, to an active mode-locked optical fiber laser generation apparatus for generating laser light having a discontinuously-changed wavelength.

Description

Fiber laser generator

The present invention relates to an optical fiber laser generator, and more particularly, to an active mode locked optical fiber laser generator in which the wavelength changes discontinuously.

Fast wavelength tunable lasers have been used for optical coherence tomography (OCT), which has attracted much attention. In the early studies of tunable lasers, dynamic variable filters such as acousto-optic modulators, polygon mirrors, and Fabry parot filters were used in the resonator. There is a limit to speeding up.

The Fourier domain mode locked laser solves the buildup type problem by synchronizing the tunable period with the resonant period. A high tunable rate of several Mhz is achieved by buffering outside the resonator and using an additional fiber amplifier.

Another method of increasing the variable speed of the wavelength is to perform active mode locking in a resonator with a very high dispersion. The active mode locking method linearly changes the frequency of the voltage signal applied to the amplitude modulator, thereby modulating the oscillation wavelength. It also changes the wavelength at high speed without a dynamic filter. However, in active mode locking, the instantaneous spectral line width and the variable wavelength range of the laser are inversely proportional to each other, which limits the performance when the laser is used in an optical coherence tomography (OCT).

Recently, lasers with discontinuous scanning of wavelengths have been studied as an alternative to overcome the bandwidth of current data acquisition systems and to extend the depth range that a tomography apparatus can reach. A wavelength step-variable laser is implemented using long fiber and harmonic mode locking.

This method shows that the depth range of the tomography apparatus can be sufficiently extended without using a bandwidth higher than that of the existing data sensing / acquisition system.

The method also ensures linearity of wavelength step changes, eliminating the need for data interpolation before Fourier transforms. There have been attempts to fabricate mode-locked lasers whose wavelengths are discontinuously step-changed for applications other than tomography devices, but no three or more steps have been implemented at wavelength intervals of less than 1 nm and specific oscillation characteristics have been given. Never had.

The present invention provides an apparatus for generating a laser having a step change of wavelength at a high speed by using a commercial optical communication element in a short resonator.

Laser generating apparatus according to an embodiment of the present invention, the amplitude modulator; A first high density wavelength multiplexer pair connected to the amplitude modulator for receiving a pulse including a plurality of wavelengths and separating and outputting the plurality of wavelengths; An optical fiber amplifier connected to the first high density wavelength multiplexer pair; And a second high density wavelength multiplexer pair connected to the optical fiber amplifier and receiving the separated plurality of wavelengths and combining the plurality of wavelengths into one pulse. It includes.

And a first output combiner disposed between the first high density wavelength multiplexer pair and the optical fiber amplifier.

And a second output combiner disposed between the second high density wavelength multiplexer pair and the amplitude modulator.

The optical fiber amplifier includes an erbium-doped optical fiber and a laser diode.

And a polarization regulator disposed between the second high density wavelength multiplexer pair and the second output coupler.

An interval between pulses output by the first high density wavelength multiplexer pair is greater than a width of the pulses.

The first high density wavelength multiplexer pair includes an optical fiber having a plurality of different lengths connected between the first high density wavelength multiplexer, the second high density wavelength multiplexer, and the first high density wavelength multiplexer and the second high density wavelength multiplexer.

The first high density wavelength multiplexer pair receives a pulse including the plurality of wavelengths and distributes the pulse to the optical fiber.

The optical fiber is disposed lengthwise between two high density wavelength multiplexers.

The order of the output wavelength is inversely proportional to the length of the optical fiber.

The second high density wavelength multiplexer pair includes a first high density wavelength multiplexer, a second high density wavelength multiplexer, and a plurality of different lengths of optical fibers connected between the first high density wavelength multiplexer and the second high density wavelength multiplexer.

The second high density wavelength multiplexer pair receives the plurality of separated wavelengths and combines the plurality of wavelengths into one pulse using the optical fiber.

The optical fiber is disposed lengthwise between two high density wavelength multiplexers.

The length of the optical fiber is inversely proportional to the order of the plurality of separated wavelengths.

The laser generating apparatus according to an embodiment of the present invention may generate a laser having a step change of wavelength at high speed by using a commercial optical communication device in a short resonator.

1A to 1C are diagrams schematically illustrating an apparatus for generating a step wavelength tunable laser according to an embodiment of the present invention.

2 is a view showing a step wavelength tunable laser generating apparatus according to an embodiment of the present invention.

3 is a view showing the operation of the amplitude modulator of the laser generating apparatus according to an embodiment of the present invention.

4 is a diagram illustrating characteristics of a mode-locked single wavelength laser for each channel.

FIG. 5 shows the optical spectrum, pulse waveform and radio frequency spectrum of each of the mode locked pulses when a modulation frequency of 3.12134 Mhz is used.

6 is a diagram showing an oscilloscope waveform of a wavelength when three wavelengths are simultaneously locked in mode.

FIG. 7 is a diagram showing an optical spectrum and a pulse waveform when three wavelength channels oscillate simultaneously.

8 is a diagram illustrating a noise spectrum in three channels.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and only the present embodiments are intended to complete the disclosure of the present invention, and the general knowledge in the art to which the present invention pertains. It is provided to fully convey the scope of the invention to those skilled in the art, and the present invention is defined only by the scope of the claims. Thus, in some embodiments, well known process steps, well known device structures and well known techniques are not described in detail in order to avoid obscuring the present invention. Like reference numerals refer to like elements throughout.

The spatially relative terms " below ", " beneath ", " lower ", " above ", " upper " It may be used to easily describe the correlation of a device or components with other devices or components. Spatially relative terms are to be understood as including terms in different directions of the device in use or operation in addition to the directions shown in the figures. For example, when flipping a device shown in the figure, a device described as "below" or "beneath" of another device may be placed "above" of another device. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device can also be oriented in other directions, so that spatially relative terms can be interpreted according to orientation.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In this specification, the singular also includes the plural unless specifically stated otherwise in the phrase. As used herein, “comprises” and / or “comprising” refers to the presence of one or more other components, steps, operations and / or elements. Or does not exclude additions.

Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used in a sense that can be commonly understood by those skilled in the art. In addition, the terms defined in the commonly used dictionaries are not ideally or excessively interpreted unless they are specifically defined clearly.

Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1A to 8.

1A to 1C are diagrams schematically illustrating an apparatus for generating a step wavelength tunable laser according to an embodiment of the present invention.

Referring to FIG. 1A, the step wavelength tunable laser generator 100 may include an optical fiber amplifier 110, first and second high density wavelength multiplexers (DWDMs) 101 and 103, an amplitude modulator 120, and a first modulator 120. And a first and second output combiner (OC) 105, 106.

The first high density wavelength multiplexer pair 101 is connected to the first output combiner 105, and the first output combiner 105 is connected to the optical fiber amplifier 110. The optical fiber amplifier 110 is connected to the second high density wavelength multiplexer pair 103, and the second high density wavelength multiplexer pair 103 is connected to the second output combiner 106. The second output coupler 106 is also connected to the amplitude modulator 120.

The first high density wavelength multiplexer 101 includes N channels and selects a laser wavelength. The first high density wavelength multiplexer 101 distributes different laser wavelengths into N different length optical fibers. The second high density wavelength multiplexer 103 combines the dispersed N wavelengths into one optical fiber.

One pulse is input to the first high density wavelength multiplexer 101, and the pulse including the plurality of wavelengths passes through the first high density wavelength multiplexer 101 and becomes N pulses that are completely separated. At this time, the optical time delay between adjacent pulses having different wavelengths in the first high density wavelength multiplexer 101 is longer than the width of the pulse.

Pulses having different wavelengths passing through the first high density wavelength multiplexer 101 pass through the optical fiber amplifier 110 at different times and gain is amplified. When the gain recovery time of the optical fiber amplifier 110 is shorter than the interval between pulses, gain competition between adjacent pulse components having different wavelengths may be avoided.

The second high density wavelength multiplexer 103 receives the pulses output from the optical fiber amplifier 110 to accurately compensate for the optical delay difference generated in the first high density wavelength multiplexer 101 and to make the resonator lengths of all the wavelength components equal. do.

The signals output from the first output combiner 105 are pulses having a step change in wavelength, and the signals output from the second output combiner 106 are single pulses mixed with different wavelengths.

The frequency of the signal applied to the amplitude modulator 120 coincides with the resonant frequency so that the output pulses are mode locked. Mode lock is a resonant frequency determined by the length of the laser generating device, which means that the phase between these frequencies is kept constant to generate periodic pulse output. The amplitude modulator 120 may be a lithium-niobate amplitude modulator.

The step wavelength tunable laser generator 100 is annular and can be active mode locked at a fundamental frequency (6 Mhz). Active mode locking means forcibly mode locking from the outside using an active device.

The first and second high density wavelength multiplexers 101 and 103 simultaneously serve as static filters for selecting the oscillation wavelength and as optical delay elements for temporally separating and combining pulses having different center wavelengths. .

1B is a view showing the operation of the first high density wavelength multiplexer 101 of the laser generating apparatus according to the embodiment of the present invention.

Referring to FIG. 1B, the first high density wavelength multiplexer 101 receives a single pulse including N wavelengths. The first high density wavelength multiplexer 101 includes N different lengths of optical fibers. A single pulse including a plurality of wavelengths is divided into N pulses having N different lengths of wavelengths, and the separated N pulses are applied to N optical fibers of different lengths, respectively. The pulse applied to the optical fiber has a different time depending on the length of the optical fiber. That is, pulses applied to the shortest optical fiber are separated first and pulses applied to the longest optical fiber are separated last.

N pulses applied to N different lengths of optical fibers are again applied to one optical fiber. At this time, N pulses have an optical time delay longer than the width of the pulse.

1C is a view showing the operation of the second high density wavelength multiplexer 103 of the laser generating apparatus according to the embodiment of the present invention.

Referring to FIG. 1C, the second high density wavelength multiplexer 103 includes N different lengths of optical fibers. The second high density wavelength multiplexer 103 receives N pulses having wavelengths of different lengths. N pulses have an optical time delay longer than the width of the pulse and are applied to N different lengths of optical fibers, respectively.

Pulses applied to the optical fiber are delayed at different times according to the optical fiber length. In other words, the pulse applied first is applied to the longest optical fiber and the pulse applied last is applied to the shortest optical fiber. In this way, N pulses applied to N different lengths of optical fibers are again applied to one optical fiber and combined into a single pulse.

2 is a view showing a step wavelength tunable laser generating apparatus according to an embodiment of the present invention.

Referring to FIG. 2, the step-wavelength laser generator includes a fiber amplifier, first and second high density wavelength multiplexer pairs 101 and 103, a polarization controller (PC) 107, and first and second outputs. Couplers 105, 106, isolators 108, and intensity modulator (IM) 120.

The first high density wavelength multiplexer pair 101 is connected with a first output combiner 105, the first output combiner 105 is connected with an isolator 108, and the isolator 108 is connected with an optical fiber amplifier. The optical fiber amplifier is connected with the second high density wavelength multiplexer 103 pair, the second high density wavelength multiplexer 103 pair is connected with the polarization regulator 107 and the polarization regulator 107 is connected with the second output combiner 106. And a second output coupler 106 is coupled with the amplitude modulator 120.

The first high density wavelength multiplexer pair 101 includes four optical fibers each having a different length and connected between the first high density wavelength multiplexer, the second high density wavelength multiplexer, and the first and second high density wavelength multiplexers, respectively. . The four optical fibers also have the same number between the first high density wavelength multiplexer and the second high density wavelength multiplexer.

The second high density wavelength multiplexer pair 103 includes four optical fibers each having a different length and connected between the first high density wavelength multiplexer, the second high density wavelength multiplexer, and the first and second high density wavelength multiplexers, respectively. . The four optical fibers also have the same number between the first high density wavelength multiplexer and the second high density wavelength multiplexer.

As the number of optical fibers included in the first high density wavelength multiplexer pair 101 increases, the length of the optical fiber becomes longer. As the number of optical fibers included in the second high density wavelength multiplexer pair 103 increases, the length of the optical fiber becomes shorter.

Therefore, the optical fibers having the same number of the first high density wavelength multiplexer pair 101 and the second high density wavelength multiplexer pair 103 have a corresponding relationship.

For example, the wavelength applied to the first optical fiber shared by the first high density wavelength multiplexer pair 101 and the second high density wavelength multiplexer pair 101 is the first of the second high density wavelength multiplexer pair 103. It is applied to the first optical fiber shared by the high density wavelength multiplexer and the second high density wavelength multiplexer. Therefore, one pulse including a plurality of wavelengths passes through the first high density wavelength multiplexer pair 101 and is separated into a plurality of pulses, and passes through the second high density wavelength multiplexer pair 103 and is combined into one pulse.

The isolator 108 allows the signal output from the first high density wavelength multiplexer 101 to proceed in only one direction.

The optical fiber amplifier includes an erbium doped fiber (EDF) 112 and a laser diode 111 (LD) 111. The erbium-doped optical fiber 112 is combined with a laser diode 111 having a length of 3 m and a wavelength of 980 nm, and pumped by the laser diode 111.

The optical fiber amplifier 110 may be an optical fiber amplifier having fast gain recovery characteristics such as a semiconductor optical amplifier (SOA).

The use of erbium-doped fiber lasers has effects such as gain competition and relaxation vibrations.

Each of the first and second high density wavelength multiplexers 101, 103 pairs comprises two commercially available high density wavelength multiplexers. The high density wavelength multiplexer has four channels spaced at 100 GHz. The central wavelength of each of the four channels is 1554.13 nm, 1554.94 nm, 1555.75 nm, 1556.56 nm. The 3-dB transmission bandwidth in each channel is about 0.11 nm.

The total length of the laser generating apparatus according to the embodiment of the present invention is about 31m including the length of the erbium-doped optical fiber, which corresponds to the resonance frequency of the laser of 6.3Mhz and the laser generator round trip time of 160ns. The time delay difference between channel 1 and channel 2 in the first high density wavelength multiplexer pair 101 is about 18.0 ns, and about 14.8 ns between channel 2 and channel 3. Thus, the optical fiber spacing between adjacent channels of the first high density wavelength multiplexer pair 101 may be about 3m to 4m. The second high density wavelength multiplexer pair 103 is adjusted within a range of 50 ps to compensate for the interchannel time delay caused by the first high density wavelength multiplexer pair 101. This means that the difference in resonator length seen by the three wavelength components is within 1 cm. The inter-channel time delay difference in the first high density wavelength multiplexer 101 may be set longer than 1.9 ns, which is a theoretically predicted pulse width by the Kuizenga-Siegman formula.

The amplitude modulator 120 includes a function generator 122 and a power supply 123.

The function generator 122 supplies a square wave voltage signal modulated by the amplitude modulator 120.

The power supply 123 supplies power for driving the amplitude modulator 120.

Pulses of different center wavelengths are separated through the optical fiber amplifier 110 including the erbium-doped optical fiber, and the time delay interval between the pulses may be longer than the gain recovery time of the amplifier.

Lithium niobate magenent-type amplitude modulators can be used for active mode locking of the laser. Since the amplitude modulator operates at a single polarization, the polarization regulator 107 is used to maximize the transmission of light through the amplitude modulator. Since the loss of the laser generating apparatus 100 depends on the polarization, all wavelengths should be the same polarization. Polarization control is achieved by changing the birefringence of the optical fiber.

In FIG. 2, a method of optimizing birefringence for each channel of the laser generating apparatus using only one polarization controller 107 is used, but the polarization controller 107 may be connected to all channels of the high density wavelength multiplexer pair.

3 is a view showing the operation of the amplitude modulator of the laser generating apparatus according to an embodiment of the present invention.

The graph 201 at the upper left shows the relationship between voltage and transmittance.

Graph 201 has the maximum transmittance at voltage V _b . Transmittance refers to the ratio of light to input. For example, when the input light is output without loss, the transmittance is 1, and when 50% is output, the transmittance is 0.5.

The graph at the bottom left shows the square wave voltage signal applied to the amplitude modulator.

Referring to the graph at the lower left, the horizontal axis represents voltage and the vertical axis represents time. The square wave voltage signal has a rise or fall time of about 18 ns and is applied to the modulator with an amplitude of 2.5 V in a 2 T _R period. Since T _R is equal to the time the laser reciprocates the resonator, the mode lock of the laser occurs. The square wave voltage signal applied to the modulator is a DC biased voltage and is set such that the rising edge and the falling edge round the maximum transmission voltage of the amplitude modulator at the same time interval. By applying a square wave voltage signal oscillating around the voltage V _b is the transmittance is maximum in the modulator it is possible to reduce the width of the pulse output.

When the frequency of the square wave applied to the amplitude modulator is equal to the resonance frequency of the laser generator, the phase between the longitudinal modes of the laser generator is constant and a mode locked pulse is generated. The longitudinal mode refers to a resonance frequency determined by the length of the laser generator.

The graph in the upper right corner shows that the square wave voltage signal applied to the amplitude modulator is modulated. Referring to the graph at the upper right, the square wave voltage signal 203 applied to the amplitude modulator is modulated by the amplitude modulator and has a transmission peak having a period of T _R.

The background loss of the amplitude modulator is about 4.17 dB and the half width of the transmission waveform 202 may be about 16 ns. This is similar to the rise or fall time of a square wave.

Since Vπ is 4V, the minimum transmittance is calculated to be about 35%. The two output couplers 105, 106 may use 5% fiber coupler.

4 is a diagram illustrating characteristics of a mode-locked single wavelength laser for each channel. 2 and 4, the polarization regulator such that the modulation frequency of the square wave input to the modulator is optimized, only one wavelength signal is selected, and the selected signal has the clearest pulse shape in order to realize the best mode lock mode. 107 is adjusted. The laser output signal is observed using an optical spectrum analyzer, a light detector with a bandwidth of 12 GHz, and a digital oscilloscope with a bandwidth of 1 GHz.

(a) to (c) show the light spectra when mode locked in channels 1 to 3, respectively. The horizontal axis of the light spectrum graph represents wavelength and the vertical axis represents output.

The optimized modulation frequencies for channels 1 to 3 are 3.121790, 3.120890 and 3.120893 MHz, respectively. Graphs 407, 408, and 409 shown inside of (a) to (c) show fitting the spectra near the spectral peaks with Gaussian curves. For reference, the center wavelengths of each channel are 1554.25, 1555.00, and 1555.80 nm, and the 3-dB spectral widths are 0.24 nm, 0.14 nm, and 0.15 nm, respectively. The spectra in (a) to (c) represent the spectra in the time domain calculated by averaging 255 samples.

(d) to (f) show the pulse waveforms 404, 405, 406 and the radio frequency spectrum 410, 411, 412 of each channel. The horizontal axis of the frequency spectrum graph represents time and the vertical axis represents intensity in units of V. FIG. The black dots represent the data and the red lines represent the approximation of the black dots in a Gaussian form.

The average pulse width of each channel is 1.4ns, 1.0ns, and 1.2ns, which is similar to Gaussian form. The average pulse width may be shorter than the pulse width of 1.93 ns, which is calculated assuming sine wave amplitude modulation.

This means that the amplitude modulation waveform may be more sharp than when sine wave amplitude modulation is assumed. The quality of the mode lock may also be sensitive to polarization control. Each wavelength channel operates properly at each optimized amplitude frequency.

The graph at the upper right of (d) to (f) shows the radio frequency spectrum.

The radio frequency spectrum represents the radio frequency spectrum of the laser. The peak frequency interval of the radio frequency spectrum corresponds to the frequency of the signal applied to the amplitude modulator. As the frequency increases, the signal strength decreases and the rate of change of the signal to frequency increases. The smaller the value, the shorter the pulse width. This means that the wider the range of the radio frequency spectrum, the better the mode lock.

The radio frequency spectrum maintains a flat height up to about 300 MHz, which means that mode lock works well.

If three wavelengths are oscillated simultaneously, one modulation frequency should be used that is not optimized for any one channel. Detuning at the optimized modulation frequency results in degraded laser performance.

Thus, one frequency whose frequency difference is similar in all channels can be selected to analyze the effects of frequency shifting on a single laser wavelength.

5 shows the optical spectrum 501, 502, 503, pulse waveforms 504, 505, 506 and the radio frequency spectrum 510, 511, 512 of each of the mode locked pulses when a modulation frequency of 3.12134 Mhz is used. Drawing.

2 and 5, the frequency shifts of channels 1 to 3 are 900 Hz, -900 Hz, and -894 Hz, respectively. The frequency output from the amplitude modulator is twice the frequency applied to the amplitude modulator. For example, if the frequency shifted from the optimized modulation frequency is 450 Hz different from the optimized modulation frequency, the output frequency has a difference of 900 Hz from the frequency at which the optimized modulation frequency is applied to the amplitude modulator and output. The spectra of (a) to (c) indicate that only one wavelength is oscillated using the polarization controller 107. The center wavelengths of the spectra of each channel approximated by a Gaussian curve are 1554.20 nm, 1555.05 nm, and 1555.95 nm, respectively, and the 3-dB spectral widths are 0.15 nm, 0.09 nm, and 0.17 nm, respectively. (d) to (f) show the pulse waveforms 504, 505 and 506 and the radio frequency spectrums 510, 511 and 512 of each channel. The horizontal axis of the frequency spectrum graph represents time and the vertical axis represents intensity in units of V. The black dots represent the data and the red lines represent the approximation of the black dots in a Gaussian form.

Referring to (d) to (f), the pulse waveform is a shape slightly deviating from the ideal Gaussian form. The widths of the pulses for the channels 1 to 3 are 1.2, 1.4, and 1.2ns, respectively, which are similar to the widths of the pulses for the channels 1 to 3 of FIGS. 4 (d) to (f). The degradation of mode locked pulses can also be caused by polarization mismatches that occur outside of the optimized modulation frequency or in the process of suppressing other wavelength components with one polarization controller.

Graphs 510, 511 and 512 in the upper right of (d) to (f) show the radio frequency spectrum. Comparing the radio frequency spectrum with Figs. 4 (d) to 4 (f) confirms the degraded mode locking performance.

6 is a diagram showing an oscilloscope waveform of a wavelength when three wavelengths are simultaneously locked in mode.

2 and 6, (a) shows the output at the first output combiner 105 when three wavelengths are simultaneously locked in mode and (b) is the second output when the three wavelengths are mode locked at the same time. A diagram showing the output at the combiner 106.

(a) to (b) show oscilloscope waveforms for the outputs of the first output combiner 105 and the second output combiner 106 when the modulation frequency is 3.12134 Mhz. The three pulses that simultaneously passed through the modulator are completely separated while passing through the first high density multiplexer pair 101 and the fiber amplifier 110. The time interval between the three pulses is 18ns between channel 1 and channel 2 and 14.8ns between channel 2 and channel 3.

7 is a diagram showing an optical spectrum and a pulse waveform when the wavelengths in three channels oscillate simultaneously.

Referring to FIG. 7, (a) shows the light spectrum 701 when the wavelengths in three channels oscillate simultaneously. The horizontal axis in (a) represents the wavelength and the vertical axis represents the output.

The center wavelength of each channel of the Gaussian approximated spectrum is 1554.20 nm in channel 1, 1555.05 nm in channel 2, and 1555.95 nm in channel 3, and the corresponding 3 dB spectral widths are 0.04 nm, 0.08 nm, and 0.07 nm, respectively. (b) through (d) show the pulse waveforms 702, 703, 704 and the radio frequency spectrum 705, 706, 707, indicating that each wavelength signal has been filtered after passing through a channel suitable for another external high density wavelength multiplexer. ). The horizontal axis in (b) to (d) represents time and the vertical axis represents intensity in units of V. The red lines in (b) to (d) represent ideal Gaussian waveforms and the black dots represent data. The pulse waveforms 702, 703, 704 and the radio frequency spectrums 705, 706, 707 of (b) to (d) are out of the ideal Gaussian waveform, and when the three wavelengths simultaneously oscillate, Waveform and radio frequency spectrum results may vary.

If more channels are available, individual polarization controllers 107 can be used for each channel inside the pair of high-density wavelength multiplexers, or a laser-generated device can be manufactured with polarization-maintaining fibers to prevent the pulse and frequency spectra from deviating from the ideal Gaussian waveform. have.

8 is a diagram illustrating a noise spectrum in three channels.

Referring to FIG. 8, the amplitude and phase noise of each channel are measured using the noise spectrum, and the characteristics of the noise spectrum are analyzed. The first peak of the radio frequency spectrum and the spectrum near the higher harmonic peaks are compared, and for each channel, the spectrum is measured for a single sideband noise, i.e. in the range of 100 Hz to 500 kHz from the peak frequency, and the RMS fluctuations in amplitude and timing. Is expected. The first and 40th radio frequency spectral peaks are used, with frequency offsets below 5 kHz measured at 30 Hz resolution and frequency offsets below 500 kHz with 10 kHz resolution.

The horizontal axis of each graph of FIG. 8 represents frequency offset and the vertical axis represents single sideband noise. The smaller the number, the less noise. That is, -40 means less noise than -60. (a)-(c) indicate when a single wavelength oscillates at an optimized modulation frequency, and (d)-(f) indicate when a single wavelength oscillates at a shifted frequency. (g)-(i) indicates Shown when three wavelengths oscillate in frequency.

Compared to a single wavelength oscillation at the optimized frequency, the noise peak at the relaxation frequency can be greater when there is a shift in the modulation frequency. As shown in (d) to (f), (g) to (i) of Fig. 8, the relaxation frequency peaks of channels 1 to 3 are around 50 kHz. Table 1 summarizes the results of measuring RMS amplitude and time fluctuations.

Channel 1 Channel 2 Channel 3 Optimum single wavelength amplitude 6% 16% 12% timing 79 ps 120 ps 110 ps Frequency shifted single wavelength amplitude 12% 16% 13% timing 68 ps 67 ps 52 ps Three frequency shifted wavelengths amplitude 24% 40% 23% timing 250 ps 240 ps 150 ps

Referring to Table 1, it is difficult to find a tendency in two cases where a single wavelength oscillates, but when three wavelengths oscillate simultaneously, amplitude and timing fluctuation increase compared to a case where a single wavelength oscillates.

Amplitude and timing fluctuations may increase due to gain competition between oscillation wavelengths, and amplitude and timing fluctuations may be solved by using an optical fiber amplifier 110 having fast gain recovery characteristics compared to pulse intervals.

Amplitude and timing fluctuations are very sensitive to the state of the polarization controller 107 and it may not be easy to find the conditions of polarization that are optimized at all wavelengths.

Since the birefringence is different in the laser generator of three wavelength components, amplitude and timing fluctuations occur. Alternatively, amplitude and timing fluctuations may occur due to nonlinear polarization rotation that occurs when the peak power of the pulse increases.

In order to implement low noise and high safety, a laser generating device made of a polarization maintaining optical fiber is preferable.

The laser generating apparatus according to an embodiment of the present invention may generate mode locked pulses of three different wavelengths with a repetition rate of 6.24 MHz near the 1555 nm center wavelength. In addition, the laser generating device may be a length of 31m.

Laser generating apparatus according to an embodiment of the present invention can generate a laser using a commercially available high density wavelength multiplexer pair that can easily implement a larger number of step wavelengths, the laser is a short generator length and relatively narrow Because of its spectral linewidth, it can be used in many applications, including tomography equipment.

Laser generating apparatus according to an embodiment of the present invention can greatly improve the operating characteristics of the laser.

The laser generating apparatus according to an embodiment of the present invention can simply implement a laser having a step change at a high speed in a short resonator using a commercial optical communication device.

In the above, embodiments of the present invention have been described with reference to the accompanying drawings, but those skilled in the art to which the present invention pertains may change to other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that it may be practiced. Therefore, it should be understood that the exemplary embodiments described above are exemplary in all respects and not restrictive.

Claims (14)

An amplitude modulator; A first high density wavelength multiplexer pair connected to the amplitude modulator for receiving a pulse including a plurality of wavelengths and separating and outputting the plurality of wavelengths; An optical fiber amplifier connected to the first high density wavelength multiplexer pair; And A second high density wavelength multiplexer pair which is connected to the optical fiber amplifier and receives the plurality of separated wavelengths and combines them into one pulse; Laser generating device comprising a. The method of claim 1, And a first output coupler disposed between the first high density wavelength multiplexer pair and the optical fiber amplifier. The method of claim 1, And a second output coupler disposed between the second high density wavelength multiplexer pair and the amplitude modulator. The method of claim 1, The optical fiber amplifier includes an erbium-doped optical fiber and a laser diode. The method of claim 3, wherein And a polarization controller disposed between the second high density wavelength multiplexer pair and the second output coupler. The method of claim 1, And a spacing between pulses output by the first high density wavelength multiplexer pair is greater than a width of the pulses. The method of claim 1, The first high density wavelength multiplexer pair includes a laser having a plurality of different lengths of optical fibers connected between the first high density wavelength multiplexer and the second high density wavelength multiplexer and the first high density wavelength multiplexer and the second high density wavelength multiplexer. Generating device. The method of claim 7, wherein And the first high density wavelength multiplexer pair receives a pulse including the plurality of wavelengths and distributes the pulse to the optical fiber. The method of claim 7, wherein And the optical fiber is disposed in length between two high density wavelength multiplexers. The method of claim 9, The order of the output wavelength is inversely proportional to the length of the optical fiber. The method of claim 1, The second high density wavelength multiplexer pair includes a laser having a plurality of different lengths of optical fibers connected between the first high density wavelength multiplexer, the second high density wavelength multiplexer, and the first high density wavelength multiplexer and the second high density wavelength multiplexer. Generating device. The method of claim 11, And the second high density wavelength multiplexer pair receives the plurality of separated wavelengths and combines them into one pulse using the optical fiber. The method of claim 11, And the optical fiber is disposed in length between two high density wavelength multiplexers. The method of claim 13, The length of the optical fiber is inversely proportional to the order of the plurality of separated wavelengths.

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