WO2023279330A1

WO2023279330A1 - Coherent optical spectrum analysis

Info

Publication number: WO2023279330A1
Application number: PCT/CN2021/105270
Authority: WO
Inventors: Le Nguyen Binh; Zhengqing YE; Xingyun Long; Zheng Zhang
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2021-07-08
Filing date: 2021-07-08
Publication date: 2023-01-12
Anticipated expiration: 2024-01-08

Abstract

A device (1) for spectral analysis of an optical signal (9) comprises a signal generation unit (2) and a signal analysis unit (3). The signal generation unit (2) is configured to generate a modulated continuous-wave, CW, laser signal (7) based on a radio frequency, RF, oscillation (f), and tune a dominant spectral line (71) of the modulated laser signal (7) across the spectrum of the optical signal (9) with a spectral resolution of less than 100 Hz. The signal analysis unit (3) is configured to analyze the spectrum of the optical signal (9) based on the modulated laser signal (7). This achieves optical spectrum analysis with high resolution and high sensitivity.

Description

COHERENT OPTICAL SPECTRUM ANALYSIS

Technical Field

The present disclosure relates generally to the field of signal analysis, and more particularly to a device as well as a method for spectral analysis of an optical signal.

Background Art

Modern communication systems and networks, especially the multiple channels in wireless and optical communications, demand a tremendous increase in information capacity. This fuels deployment of further dimensions of communications, such as polarization or sideband/subcarrier modulation in fiber-optical communications.

In view of the apparent convergence of wireless and optical communications in applications such as radio over fiber, wherein wireless information channels are carried over fiber-optical transmission lines, a bandwidth of the information channels is much smaller than the optical carrier frequency. A monitoring of the spectrum of these information channels requires fast, accurate, and high- resolution optical spectrum analysis.

Summary

It is an object to overcome these and other drawbacks, and to provide a device for spectral analysis of an optical signal. In particular, the device should be capable of monitoring a high-speed complex wideband optical data channel transmission, including polarization-multiplexed transmission and multi-symbol modulation in long haul optical transmission systems, and of narrow-band data channels carried on Radio Frequency (RF) carriers over fiber for 5^th generation or 6^th generation (5G or 6G) wireless networking in radio-over-fiber transmission systems. The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, a device for spectral analysis of an optical signal is provided. The device comprises a signal generation unit and a signal analysis unit. The signal generation unit is configured to generate a modulated continuous-wave (CW) laser signal based on a radio frequency, RF, oscillation, and tune a dominant spectral line of the modulated laser signal across the spectrum of the optical signal with a spectral resolution of less than 100 Hz. The signal analysis unit is configured to analyze the spectrum of the optical signal based on the modulated laser signal.

Appropriate modulation of the laser signal with an RF oscillation, which suppresses an optical carrier of the laser signal, achieves a laser signal having a single spectral line.

The suppression of the carrier and the second sideband eliminates spurious mixing of optical spectral components, and hence unwanted coherent products. This may result in significant noise suppression and improvement in dynamic range, which may be in excess of 80 dB.

In a possible implementation form, the signal generation unit may be configured to tune the dominant spectral line of the modulated laser signal across the spectrum of the optical signal with a spectral resolution of less than 10 Hz.

The lower limit of the RF oscillation may reach the lower limit of the linewidth of the laser signal, and the upper limit may reach 60 to 90 GHz and even higher depending on how the RF oscillation is generated, which meets the optical channel bandwidths typically used in fiber-optical wavelength-division multiplexed (WDM) transmission.

In a possible implementation form, the optical signal may comprise a number of subcarrier multiplexes (SCMs) of signals having a respective bandwidth of up to 500 MHz.

This enables a spectrum analysis of SCM-multiplexed radio over fiber (ROF) signals, wherein wireless information channels are carried over fiber-optical transmission lines. In a possible implementation form, the optical signal may comprise a dense wavelength division multiplex (DWDM) of the number of SCMs.

This enables a spectrum analysis of DWDM- multiplexed optical signals.

In a possible implementation form, the signal generation unit may be configured to tune the dominant spectral line of the modulated laser signal by tuning the CW frequency of the modulated laser signal and/or tuning the frequency of the RF oscillation.

Tuning a CW frequency of the laser signal and/or tuning a frequency of the RF oscillation sweeps the single spectral line across the optical spectrum of interest, thereby achieving accurate optical spectrum analysis in particular in the sideband/subcarrier dimension.

In a possible implementation form, the signal generation unit may comprise a modulation unit configured to modulate the CW laser signal in dependence of the RF oscillation.

In a possible implementation form, the signal generation unit may further comprise a Hilbert transformer unit configured to impart a phase shift of p/2 between in-phase and quadrature-phase components of the RF oscillation.

This achieves / introduces a 90 degree phase shift over a frequency range with substantially constant amplitude for all frequencies.

In a possible implementation form, the signal generation unit may further comprise a phase tuning unit configured to tune the phase shift between the in-phase and quadrature-phase components of the RF oscillation.

This splits the orthogonal electrical components in the optical domain for appropriate detection. In a possible implementation form, the modulated laser signal may comprise an optical single sideband suppressed-carrier (SSB-SC) signal.

A laser signal having a single spectral line, such as an optical SSB-SC signal, enables an accurate and high-resolution optical spectrum analysis.

In a possible implementation form, the signal analysis unit may comprise a demodulation unit configured to demodulate in-phase and quadrature-phase components of the optical signal.

In a possible implementation form, the optical signal may comprise a polarization division multiplex (PDM) of DWDMs having orthogonal states of polarization (SOPs), and the signal analysis unit may further comprise a polarization rotation unit configured to rotate an SOP of the modulated laser signal between the orthogonal SOPs of the optical signal.

This enables a spectrum analysis of PDM-multiplexed optical signals.

In a possible implementation form, the demodulation unit may further comprise a coupler unit configured to split the polarization-rotated modulated laser signal into separate identical beams; a polarizing beam splitter (PBS) unit configured to split the optical signal into separate beams having orthogonal SOPs; a plurality of polarization alignment units configured to align the respective SOP of the beams of the optical signal and the SOP of the beams of the modulated laser signal; and a plurality of 90° optical hybrid units configured to combine (i.e., coherently mix) the polarization- aligned beams of the optical signal and the modulated laser signal.

In a possible implementation form, the demodulation unit may further comprise a plurality of balanced optical detection units configured to demodulate (i.e., perform homodyne detection of) the respective in-phase and quadrature-phase components of the combined polarization-aligned beams within a respective detection bandwidth of less than 100 kHz.

Coherent mixing of the optical spectrum of interest and the laser signal followed by narrowband homodyne detection achieves accurate optical spectrum analysis at the single spectral line with high sensitivity and a high resolution of less than 10 Hz.

In a possible implementation form, the signal analysis unit may further comprise a digital processing unit configured to sample the demodulated in-phase and quadrature-phase components for conversion into one or more equivalent optical spectra of the optical signal.

This enables inspection and evaluation of the analyzed optical spectrum, e.g. by a user, and/or further software-based spectrum analysis.

According to a second aspect, a method for spectral analysis of an optical signal is provided. The method comprises steps of: generating a modulated CW laser signal based on an RF oscillation; tuning a dominant spectral line of the modulated laser signal across the spectrum of the optical signal with a spectral resolution of less than 100 Hz; and analyzing the spectrum of the optical signal based on the modulated laser signal.

According to a third aspect, a computer program is provided, comprising executable instructions which, when executed by a processor, cause the processor to perform the method of the second aspect or any of its embodiments.

Polarization diversity splitting of the optical spectrum of interest achieves accurate optical spectrum analysis of the polarization dimension.

This may simplify and expedite a monitoring of high-speed complex wideband optical data channel transmission, including polarization-multiplexed transmission and multi-symbol modulation such as QPSK or QAM, in long haul optical transmission systems, and of narrow-band data channels carried on RF carriers over fiber for 5G/6G networking in radio- over-fiber transmission systems.

The technical effects and advantages described above equally apply to the device and the method for spectral analysis of an optical signal having corresponding features as well as to the computer program. Brief Description of Drawings

The above-described aspects and implementations will now be explained with reference to the accompanying drawings, in which the same or similar reference numerals designate the same or similar elements.

The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to those skilled in the art.

FIG. 1 illustrates schematically a device for spectral analysis of an optical signal in accordance with the present disclosure;

FIG. 2 illustrates an exemplary optical signal of interest;

FIG. 3 illustrates schematically an exemplary signal generation unit in accordance with the present disclosure of the device of FIG. 1;

FIG. 4 illustrates schematically an exemplary signal analysis unit in accordance with the present disclosure of the device of FIG. 1; and

FIG. 5 illustrates a method for spectral analysis of an optical signal in accordance with the present disclosure.

Detailed Descriptions of Drawings

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of this disclosure or specific aspects in which the embodiments may be used. It is understood that the embodiments may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the embodiments of this disclosure is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding apparatus or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 illustrates schematically a device 1 for spectral analysis of an optical signal 9 in accordance with the present disclosure.

The device 1 comprises a signal generation unit 2 and a signal analysis unit 3.

The signal generation unit 2 may receive a CW laser signal 6 generated by a fixed or tunable laser 5 (see FIG. 3 below) and provide a modulated CW laser signal 7 to the signal analysis unit 3.

The signal analysis unit 3 may receive this modulated CW laser signal 7 for use as a local oscillator in coherent demodulation/detection as well as an optical signal 9 of interest, which is to be analyzed in terms of its optical spectrum. The signal generation unit 2 and the signal analysis unit 3 will be explained in more detail in connection with FIGs. 3 and 4 below.

FIG. 2 illustrates an exemplary optical signal 9 of interest.

As shown in an upper portion of FIG. 2, the optical signal 9 may comprise a number of signals 81 having a respective bandwidth of up to 500 MHz, for example.

In particular, these signals 81 may be subcarrier multiplexed. As used herein, subcarrier multiplexing may refer to a method for combining (multiplexing) a plurality of electrical RF subcarrier signals into an electrical RF composite signal. More specifically, a baseband data of the respective signal 81 may be modulated on a respective RF subcarrier fi-.fu and subsequently combined, for example using an RF power combiner (not shown). This way each signal 81 occupies a different portion of the electrical RF spectrum. As used herein, a subcarrier multiplex, SCM, 8 may refer to the electrical RF composite signal mentioned above.

As shown in a lower portion of FIG. 2, the optical signal 9 may comprise a number of such SCMs 8, in particular a dense wavelength division multiplex, DWDM, 9 of the SCMs 8. As used herein, dense wavelength division multiplexing may refer to a method for combining (multiplexing) a plurality of optical carrier signals into an optical composite signal for transmission via an optical fiber. More specifically, the respective electrical RF composite signal 8 may be modulated on a respective optical center frequency V_J..V_N and subsequently combined, for example using a DWDM power combiner (not shown). This way each signal 81 occupies a different portion of the optical spectrum surrounding the respective optical center frequency

AS used herein, a dense wavelength division multiplex, DWDM, 9 may refer to the optical composite signal mentioned above.

Not shown, but mentioned here is that the optical signal 9 may comprise up to two of such DWDMs 9 within a same optical spectrum, in particular a polarization division multiplex, PDM, of DWDMs having orthogonal states of polarization, SOPs. As used herein, polarization (a.k.a. polarisation) may generally refer to a transverse orientation of the oscillations of propagating electromagnetic waves, such as light waves, with respect to their propagation direction, an SOP may refer to a particular transverse orientation, and orthogonal SOPs may refer to two particular transverse orientations substantially enclosing a right angle. As used herein, polarization division multiplexing may refer to a method for combining (multiplexing) two linearly polarized optical signals having orthogonal SOPs into an optical composite signal including both SOPs, for example using a polarization beam combiner. As used herein, a polarization division multiplex, PDM, may refer to the optical composite signal including both SOPs.

Those skilled in the art may appreciate that the bandwidth of individual signals 81 of up to 500 MHz is much smaller than the optical center frequencies

of the DWDM 9. For example, the C(onventional) transmission band, where optical fibers have a small transmission loss, extends from 1.530 to 1.565 nm which corresponds to optical center frequencies of 195,94 to 191,56 THz.

Typical wavelength resolutions of tunable lasers of e.g. 1 pm correspond to around 250 MHz, which is in the order of the bandwidth of individual signals 81 of up to 500 MHz. As such, a much higher spectral resolution is needed for accurate optical spectrum analysis.

FIG. 3 illustrates schematically an exemplary signal generation unit 2 in accordance with the present disclosure of the device 1 of FIG. 1.

The signal generation unit 2 is configured to receive a CW laser signal 6 having a CW frequency Vo and generate a modulated continuous-wave, CW, laser signal 7 based on a radio frequency, RF, oscillation.

To this end, the signal generation unit 2 may comprise a modulation unit 21 configured to modulate the received CW laser signal 6 (and thus the modulated CW laser signal 7, too) in dependence of the RF oscillation. For example, the modulation unit 21 may comprise a dual-polarized Quadrature Phase Shift Keying type in which both orthogonal SOPs can be modulated.

More specifically, the signal generation unit 2 may comprise an RF oscillator unit 23 configured to generate the RF oscillation at an RF frequency / and may comprise a Hilbert transformer unit 22 applied to the RF oscillator unit 23 and configured to impart a phase shift of p/2 between in-phase I and quadrature-phase Q components of the RF oscillation. As used herein, a Hilbert transformer unit may refer to any circuit that achieves / introduces a 90 degree phase shift over a frequency range, with substantially constant amplitude for all frequencies.

One sideband of the received CW laser signal 6 may be suppressed by biasing modulator units (not shown) of the modulation unit 21 such that the phase shift of p/2 between to two branches of the light waves can be achieved.

The signal generation unit 2 may further comprise a phase tuning unit 24 configured to split the orthogonal electrical components in the optical domain so that they can be detected appropriately.

As shown in a right-hand portion of FIG. 3, the modulated laser signal 7 may thus comprise an optical single-sideband suppressed-carrier, SSB-SC, signal, wherein a dominant spectral line 71 is frequency- shifted by the frequency / of the RF oscillation.

The signal generation unit 2 may be configured to tune the dominant spectral line 71 of the modulated laser signal 7 at frequency vo+f by tuning the CW frequency v_(l of the modulated laser signal 7 and/or tuning the frequency / of the RF oscillation. This allows for a pure mixing of the dominant spectral line 71 with the corresponding spectral component of the optical spectrum of interest, spectral line by spectral line, to give a coherent beating product that may subsequently be optically demodulated/detected and electrically processed.

As such, a much higher spectral resolution than the typical wavelength resolutions of tunable lasers may be achieved. In particular, the signal generation unit 2 is configured to tune the dominant spectral line 71 of the modulated laser signal 7 across the spectrum of the optical signal 9 with a spectral resolution of less than 100 Hz, and preferably less than 10 Hz.

In addition, the combined tuning of the CW frequency vo of the modulated laser signal 7 and the frequency / of the RF oscillation allows for a wideband optical spectrum analysis across more than 150 nm or 150 x 125GHz (=18.75 THz). FIG. 4 illustrates schematically an exemplary signal analysis unit 3 in accordance with the present disclosure of the device 1 of FIG. 1.

The signal analysis unit 3 is configured to analyze the spectrum of the optical signal 9 based on the modulated laser signal 7 provided by the signal generation unit 2.

The optical signal 9 and the modulated laser signal 7 are provided as an input to the signal analysis unit 3, as is depicted in the left-hand portion of FIG. 4.

For a polarization- multiplexed optical signal 9, the signal analysis unit 3 may comprise a polarization rotation unit 3 configured to rotate an SOP of the modulated laser signal 7, for example between the orthogonal SOPs of the optical signal 9. This allows for using a same linearly polarized modulated laser signal 7 for mixing with both of the orthogonal SOPs of the optical signal 9. In other words, an SOP (polarization direction) of the laser signal 7 may be rotated in such a way that it can be split into both polarized modes (X-pol, Y-pol).

The signal analysis unit 3 may comprise a demodulation unit 32 configured to demodulate in-phase I and quadrature-phase Q components of the optical signal 9.

The demodulation unit 32 may comprise a coupler unit 321 configured to split the polarization- rotated modulated laser signal 7 into separate identical beams. Alternatively, a polarizing beam splitter, PBS, unit (not shown) may be used, which may improve a sensitivity in the subsequent demodulation by up to 3dB.

The demodulation unit 32 may comprise a PBS unit 322 configured to split the optical signal 9 into separate beams having orthogonal SOPs; and a plurality of polarization alignment units 323 configured to align the respective SOP of the beams of the optical signal 9 and the (identical) SOP of the beams of the modulated laser signal 7. This may further improve a sensitivity of the subsequent demodulation, as only fully polarization-aligned signals yield the best demodulation results, whereas fully polarization-unaligned (i.e., orthogonal) signals do not mix at all. The demodulation unit 32 may comprise a plurality of 90° optical hybrid units 324 configured to combine the polarization-aligned (pairs of) beams of the optical signal 9 and the modulated laser signal 7. Each of the plurality of 90° optical hybrid units 324 may provide respective in-phase I and quadrature-phase Q components of the combined polarization-aligned beams relating to different SOPs (i.e., X-pol, Y-pol).

The demodulation unit 32 may comprise a phase controller (not shown) configured to control respective phases of the respective in-phase I and quadrature-phase Q components so that the components can separately be mixed and output of the plurality of 90° optical hybrid units 324.

The demodulation unit 32 may further comprise a plurality of balanced optical detection units 325 configured to demodulate the respective in-phase I and quadrature-phase Q components of the combined polarization-aligned beams within a respective detection bandwidth of less than 100 kHz. This detection bandwidth is relatively small in comparison with photodetection bandwidths of balanced receivers in typical DWDM transmission systems such as 10 - 20 GHz, for example, and results in a significant noise reduction as wideband noise is rejected. The detection bandwidth may be a property of the balanced optical detection units 325 or a result of narrowband low-pass filtering before detection. As used herein, balanced photodetection (or differential photodetection) may refer to a detection method wherein differences in optical power between two optical input signals are detected while common fluctuations of the optical input signals are largely suppressed. As an example, two photodiodes may be connected back to back so that their photocurrents cancel each other when they are equal, and the identity or difference in photocurrents may be evaluated.

Those skilled in the art will appreciate that combining identical frequencies of the optical signal 9 of interest and the modulated laser signal 7 (i.e., the local oscillator) represents a homodyne demodulation approach. Homodyne demodulation offers a 3 dB sensitivity improvement over heterodyne demodulation in which the combined frequencies are different.

The demodulation unit 32 may also be termed as a coherent optical spectrum analyzer, COSA. The signal analysis unit 3 may further comprise a digital processing unit 33, such as a digital signal processor, DSP, or an application-specific integrated circuit, ASIC, configured to sample the demodulated in-phase I and quadrature-phase Q components for conversion into one or more equivalent optical spectra of the optical signal 9, depending on the complexity of the optical signal 9 and the setup of the device 1.

FIG. 5 illustrates a method 4 for spectral analysis of an optical signal 9 in accordance with the present disclosure.

The method 4 comprises steps of: generating 41 a modulated CW laser signal 7 based on an RF oscillation; tuning 42 a dominant spectral line 71 of the modulated laser signal 7 across the spectrum of the optical signal 9 with a spectral resolution of less than 100 Hz; and analyzing 43 the spectrum of the optical signal 9 based on the modulated laser signal 7.

A corresponding computer program (not shown) comprises executable instructions which, when executed by a processor, cause the processor to perform the method 4 for spectral analysis of the optical signal 9.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

1. A device (1) for spectral analysis of an optical signal (9), the device (1) comprising a signal generation unit (2) configured to generate a modulated continuous-wave, CW, laser signal (7) based on a radio frequency, RF, oscillation (f), and tune a dominant spectral line (71) of the modulated laser signal (7) across the spectrum of the optical signal (9) with a spectral resolution of less than 100 Hz; and a signal analysis unit (3) configured to analyze the spectrum of the optical signal (9) based on the modulated laser signal (7).

2. The device (1) of claim 1, wherein the signal generation unit (2) is configured to tune the dominant spectral line (71) of the modulated laser signal (7) across the spectrum of the optical signal (9) with a spectral resolution of less than 10 Hz.

3. The device (1) of claim 1 or claim 2, wherein the optical signal (9) comprises a number of subcarrier multiplexes, SCMs (8), of signals (81) having a respective bandwidth of up to 500 MHz.

4. The device (1) of claim 3, wherein the optical signal (9) comprises a dense wavelength division multiplex, DWDM, of the number of SCMs (8).

5. The device (1) of any one of the preceding claims, wherein the signal generation unit (2) is configured to tune the dominant spectral line (71) of the modulated laser signal (7) by tuning the CW wavelength (vo) of the modulated laser signal (7) and/or tuning the frequency (f) of the RF oscillation.

6. The device (1) of any one of the preceding claims, wherein the signal generation unit (2) comprises a modulation unit (21) configured to modulate the CW laser signal (7) in dependence of the RF oscillation.

7. The device (1) of claim 6, wherein the signal generation unit (2) further comprises a Hilbert transformer unit (22) configured to impart a phase shift of p/2 between in- phase (I) and quadrature-phase (Q) components of the RF oscillation.

8. The device (1) of claim 7, wherein the signal generation unit (2) further comprises a phase tuning unit (24) configured to tune the phase shift between the in-phase (I) and quadrature-phase (Q) components of the RF oscillation.

9. The device (1) of any one of the preceding claims, wherein the modulated laser signal (7) comprises an optical single-sideband suppressed-carrier, SSB-SC, signal.

10. The device (1) of any one of the preceding claims, wherein the signal analysis unit (3) comprises a demodulation unit (32) configured to demodulate in-phase (I) and quadrature-phase (Q) components of the optical signal (9).

11. The device (1) of claim 10, wherein the optical signal (9) comprises a polarization division multiplex, PDM, of DWDMs having orthogonal states of polarization, SOPs; and wherein the signal analysis unit (3) further comprises a polarization rotation unit (31) configured to rotate an SOP of the modulated laser signal (7) between the orthogonal SOPs of the optical signal (9).

12. The device (1) of claim 11, wherein the demodulation unit (32) further comprises a coupler unit (321) configured to split the polarization-rotated modulated laser signal (7) into separate identical beams; a polarizing beam splitter, PBS, unit (322) configured to split the optical signal (9) into separate beams having orthogonal SOPs; a plurality of polarization alignment units (323) configured to align the respective SOP of the beams of the optical signal (9) and the SOP of the beams of the modulated laser signal (7); and a plurality of 90° optical hybrid units (324) configured to combine the polarization- aligned beams of the optical signal (9) and the modulated laser signal (7).

13. The device (1) of claim 12, wherein the demodulation unit (32) further comprises a plurality of balanced optical detection units (325) configured to demodulate the respective in-phase (I) and quadrature-phase (Q) components of the combined polarization- aligned beams within a respective detection bandwidth of less than 100 kHz.

14. The device (1) of any one of the claims 9 to 13, wherein the signal analysis unit (3) further comprises a digital processing unit (33) configured to sample the demodulated in-phase (I) and quadrature-phase (Q) components for conversion into one or more equivalent optical spectra of the optical signal (9).

15. A method (4) for spectral analysis of an optical signal (9), comprising generating (41) a modulated CW laser signal (7) based on an RF oscillation; tuning (42) a dominant spectral line (71) of the modulated laser signal (7) across the spectrum of the optical signal (9) with a spectral resolution of less than 100 Hz; and analyzing (43) the spectrum of the optical signal (9) based on the modulated laser signal (7).

16. A computer program, comprising executable instructions which, when executed by a processor, cause the processor to perform the method (4) of claim 16.