WO2004064211A1

WO2004064211A1 - Laser array

Info

Publication number: WO2004064211A1
Application number: PCT/AU2004/000046
Authority: WO
Inventors: Kamran Eshraghian; Kamal Alameh; Rainer Michalzik
Original assignee: Edith Cowan University
Current assignee: Edith Cowan University
Priority date: 2003-01-15
Filing date: 2004-01-14
Publication date: 2004-07-29
Anticipated expiration: 2005-07-15
Also published as: AU2003900172A0

Abstract

A semiconductor laser element (50) includes a first reflector (2a) and a second reflector (3), and an active region (2) provided between the first and second reflectors for providing optical amplification by stimulated electron-hole recombination. The first reflector (2a) is a distributed Bragg reflector (DBR). The second reflector (3) is a high-reflection coated microlens which establishes a semi-spherical cavity between the first and second reflectors. Also disclosed is an integrated array (1) of the semiconductor laser elements (50).

Description

"Laser Array"

Field of the Invention

This invention relates generally to devices that emit electromagnetic radiation and, in particular, a laser for generating one or more high-power surface-emitting laser beams for pumping rare-earth-doped optical amplifiers.

Background Art

Lasers have a wide range of applications. Laser types include gas, solid-state, liquid (dye), free electron, and semiconductor lasers.

In parallel with the advances made since 1962, much effort has been dedicated to semiconductor lasers, which have unique properties unavailable in other laser systems. Because of their high density of electronic states, the amplification of the semiconductor laser is enormous, of the order of hundreds per centimetre. Two laser configurations have attracted a lot of attention, namely, the edge- emitter laser and the vertical-cavity surface-emitting laser (VCSEL). Edge emitters have cavity lengths of few hundred microns, and hence their gain is high, enabling them to lase satisfactorily with quite poor mirrors (that is with reflectivities R~31%).

A VCSEL also makes use of the large gain of the excited semiconductor, but is much shorter than the edge-emitter (with cavity lengths of the order 1 micron). The active region in which the amplification of the optical field takes place usually has a thickness of only several tens of nanometres. In order to achieve lasing with such a thin layer of gain medium, high reflectivity mirrors are required - termed Distributed Bragg Reflectors (DBRs).

There are other types of semiconductor lasers, including non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor laser pumping can be electrical (by a DC or AC current), optical, or electron beam pumping.

A VCSEL structure usually consists of an active (gain) region, sandwiched between two DBRs, and is constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections).

Another type of VCSEL is the vertical external cavity surface emitting laser (VECSEL) structure, where the active region is sandwiched between an integrated DBR and an external DBR. VECSELs can be designed to have a circular, Gaussian shaped laser beam, a small divergence angle, and can generate high optical power, and hence are attractive in many applications.

VECSELs are typically optically pumped by an external high power edge-emitting laser diode. A more compact arrangement is achieved with electrical pumping. Here a current is injected across the active region, where high-energy electrons interact with photons to produce stimulated emissions, and hence optical amplification. When the optical gain is higher than the optical losses (which include both DBR and the active region losses) lasing takes place and the laser output is emitted through the external mirror.

VECSELs have a relatively small volume and can generate a large amount of power. In addition, the capability of VECSEL arrays to be monolithically integrated makes them very attractive for multi-port optical amplification. Coupling the output of each 980 nm VECSEL element to a corresponding erbium-doped fibre is now feasible through a microlens array etched directly into a glass or sapphire substrate bonded to the VECSEL array. It is also possible to deposit an Anti-Reflection (AR) coating on one of the outer surfaces of the microlens array to simultaneously provide beam shaping and external mirror reflectivity. The integration of high-power VECSEL arrays with advanced microlens arrays will open the way towards the fabrication of multi-port optical amplifiers, which are key components in many emerging applications. , Disclosure of the Invention

In accordance with the present invention, there is provided a semiconductor laser element comprising a first reflector, and a second reflector, and an active region provided between the first and second reflectors for providing optical amplification by stimulated electron-hold recombination, the second reflector being curved to thereby establish a semi-spherical cavity between the first and second reflectors.

Preferably, the curved second reflector is a microlens provided with a reflective coating.

Preferably, the laser element includes a semiconductor substrate onto which the microlens is mounted.

Preferably, the microlens is mounted onto an optical transparent substrate such as a glass or sapphire substrate attached to the semiconductor substrate.

The laser element may further include a relay means, such as a microlens or diffractive optical element, provided onto a second transparent substrate such as a glass or sapphire substrate.

Preferably, the first reflector is a distributed Bragg reflector. Furthermore, the distributed Bragg reflector comprises a multiplicity of pairs of layers of high and low refractive index layers.

Preferably, the semiconductor laser element is provided with a heat sink arranged to dissipate heat generated by current flow through the laser structure.

Preferably, the active region consists of multiple quantum, wells.

Preferably, the element includes contact means for providing excitation of the active region.

Preferably, the element includes monitoring means for monitoring the optical beam generated within the active layer. Preferably, the laser element is provided in an array, and, preferably, a two- dimensional array, each element within the array being operable to generate an individual optical beam.

Preferably, each laser element in the array is provided with contact means.

Preferably, each laser element in the array includes monitoring means.

Preferably, the array is arranged for coupling to processing means for receiving the generated optical beam from elements of the array, and arranged to receive control signals from the processing means for the elements to control generation of the optical beams. An array of the present invention is particularly well suited to coupling with fibre collimator arrays.

Thus, a semiconductor laser element, and array of such elements, can be provided wherein loss discrimination between transverse modes can be controlled thus allowing output power to be increased while maintaining fundamental mode oscillation. The present invention overcomes the low-power disadvantages of present devices.

Brief Description of the Drawings

The invention will now be described, by way of example, only, with reference to the accompanying drawings, in which:

Figures 1a, 1 b and 1c illustrate the energy band diagram for a double heterostructure semiconductor laser;

Figure 2 illustrates a typical VCSEL structure; and

Figure 3 illustrates a generic VECSEL structure of the invention. Best Mode(s) for Carrying Out the Invention

The amplification section of a semiconductor laser, is a development of the p-n junction called the double heterojunction heterostructure. The double- heterojunction consists of a layer of p-type material and a layer of n-type material with a thin layer of lower band-gap material grown epitaxially between them. Figure 1(a) shows the energy band diagrams for an unbiased double heterostructure laser, where the smaller band-gap material is of p-type. Often this layer is not intentionally doped. Figure 1(b) shows the energy band diagram where a forward bias is applied to the junction. Figure 1(c) illustrates the injection of conduction band electrons and valence band holes into the active region - known as a quantum well, where the thickness is below a certain value, for example 20 nm. The heterojunction interfaces provide a potential barrier to the further progress of both types of carriers. This trapping of the electrons and holes allows for the build-up of substantial populations of both species where they can combine radiatively to provide optical gain. Note that the refractive index of the active region is higher than those of the cladding layers, thus providing a wave- guiding mechanism for edge-emitters. This guiding mechanism is not of use to VCSELs as the cavity axis is orthogonal to the active region in these lasers.

Figure 2 shows a typical, previously proposed VCSEL structure 100. The VCSEL 100 comprises an inner cavity 103, containing a quantum well active region surrounded by cladding layers similar to Fig. 1. The inner cavity 103 is sandwiched between a p-doped DBR 102 and an n-doped DBR 104. The n- doped DBR 104 is an integrated structure provided on an n-doped Gallium

Arsenide (GaAs) substrate 105. Bias current is applied to the VCSEL 100, via contacts 106. Note that the cavity axis is orthogonal to the inner cavity 103, rather than along it. The gain per unit length is usually very high, but the active region is extremely thin, usually comprising several quantum wells with a total thickness of several tens of nanometers, and therefore the gain per pass is very low, hence, high reflectivity mirrors in the form of the DBRs 102, 104, are required. This extremely short cavity ensures that there is only one Fabry-Perot resonance that overlaps with the gain bandwidth (the wavelength range over which the inner cavity 103 provides gain), and that the VCSEL 100 is single longitudinal mode. In these lasers, any direction orthogonal to the cavity axis is termed transverse. As well as a high-Q cavity, the structure of the VCSEL 100 must also provide some means of confining the injected carriers, so that a sufficiently low threshold current density can be achieved. The most effective confinement mechanism used in current VCSELs is the oxide-confined.

In order to increase the power from the VCSEL 100, it is necessary to increase its transverse dimension. This causes higher power structures to operate in multiple transverse modes, and engineering structures to provide higher single mode power (>50 mW) is currently a topic of interest.

The DBRs 102, 104 are stacks of many pairs of semiconductor layers, in the most simple case each λ/4 thick at the design wavelength of λ, where λ is the wavelength of the optical wave in the semiconductor material. Each pair is composed of a high and low index layer and although the reflectance at each interface is small, all the reflectances add, in phase, and the net effect is a mirror with a reflectance, R, of > 99% over a stopband of some tens of nm. The resonance wavelength of the cavity is given by the separation of the DBRs 102, 104 which should be some multiple of λ/2, and most commonly has an optical length of λ. A λ-cavity has a field maximum at its centre, and it is here that the quantum well active region within the inner cavity 103 is placed, in order to maximise the overlap between the field and the carriers. The technical difficulties in growing mirror stacks such as these proved to be an early obstacle to VCSEL development. The most critical problem was the electrical resistance. Each interface between a high and low index layer is a heterojunction and thus is a potential barrier for carriers. This problem is more acute in the p-doped DBR 102, as the heavier hole mass reduces tunnelling through the barriers.

Various methods such as δ-doping, C-doping and periodically graded alloys have been used to reduce the electrical impedance and have achieved a large measure of success. However the specific resistance (i.e. the ohmic resistance times unit area) of VCSELs is still much greater than that of edge emitters. This leads to excessive heating of the structure, which is deleterious to the performance of the VCSEL 100. Efforts have been made to get around this problem altogether by not using the top DBR 102 for conduction and, instead, wafer bonding a stack of higher refractive index contrast pairs of dielectric layers to the inner cavity 103 and placing the current contacts 106 underneath. With the higher.contrast mirror pairs, fewer pairs are required to achieve sufficient reflectivity and in ultra-small devices with more highly divergent output, there is expected to be less diffraction loss in the higher contrast DBR 102.

The approach of the present invention, to improve the brightness of the emitted beam is based on replacing the bottom DBR 104 by an external curved mirror, which allows the control of the loss discrimination between transverse modes so that the active diameter, and thus the output power, can be increased while maintaining fundamental mode oscillation. Together with the integrated DBR 102, the curved mirror establishes a stable semi-spherical cavity with greatly extended length, in which the Gaussian-shaped fundamental mode has the lowest diffraction losses and the best spatial overlap with the transverse gain profile in the active layer.

Figure 3 shows a generic high-power VECSEL array 1 according to a preferred embodiment of the present invention.

The VECSEL array 1 comprises an integrated structure composed of the compound semiconductor materials GaAs, AIGaAs and InAIGaAs with a number of elements 50 - three of which are illustrated in Figure 3. Each element 50 comprises a monolithic top semiconductor DBR 2a, an inner cavity 2c, usually with a thickness of one material wavelength λ, an oxide confinement layer 2b located close to the interface between the inner cavity 2c and the top DBR 2a, as well as possibly an integrated bottom DBR (not shown) between the bottom contact layer 6a and the inner cavity 2c, where the bottom DBR has a reduced number of mirror pairs compared to the top DBR 2a.

The inner cavity 2c contains the active layer 2, which is a double heterojunction heterostructure and is composed of several InGaAs or InAIGaAs quantum wells exhibiting high gain in the 980 nm spectral region. The monolithic top DBR 2a consists of a p-doped stack of around 25 layer pairs of high and low refractive index layers, each pair comprising two quarter-wave thick layers such as GaAs and Al₀.₉oGa₀.ιoAs. The oxide confinement layer 2b, comprises a thin (-25 nm) AIGaAs layer with high Al content such as Alo.₉₈Gao.₀₂As, and is usually incorporated into the first layer pair of the top DBR 2a. Within the processing of the VECSEL array 1 , this layer 2b is partially converted into an electrically insulating oxide material by the technique of selective oxidation.

The main bottom mirror is realized by a high-reflection coated microlens 3 integrated onto a transparent substrate such as a glass or sapphire substrate 7 and attached to a - preferably undoped - GaAs semiconductor substrate 6. In the array, a number of microlenses 3 are provided on the substrate 7 - one for each element 50. This is shown clearly in Figure 3. The microlenses 3 can be directly etched onto the substrate 7. The microlens 3 provides a semi-spherical bottom mirror that collimates the laser beam, thereby preventing divergence, which can cause losses.

A laser with a mesa size ranging from 30 microns to 70 microns can be fabricated by etching circular mesas through the oxide layer 2b with wet chemical etching or dry etching such as reactive ion etching and oxidising laterally the high Al content oxide layer 2b. A second etching step provides access to a doped bottom contact layer 6a. Doping is of n-type if the top DBR 2a is p-type doped, or vice versa. A bottom ohmic contact 6b is deposited on the bottom contact layer 6a. The mesas are planarized with a dielectric material 6c such as polyimide. An opening is formed in the planarizing material 6c by means such as photolithography and a vertical metallic contact post 6d is established by electroplating a suitable material such as gold.

For each element 50, a top contact layer 6e is provided on each top DBR 2a. The top contact layer 6e is made from TiPtAu for p-type contact or from AuGeNiAu for n-type contact. An insulating layer 11 is then provided above the top contact layers 6e i.e. on top of the semiconductor surface. The insulating layer 11 can be made of any suitable material, such as polyimide. Metal lines 10 for feeding the operating current to the individual elements are formed on the insulating layer 11 on top of the semiconductor surface and fan out toward the sides where contact to external power control circuits can be made.

Indium solder bumps 5 are provided above the insulating layer 11. The bumps 5 ensure efficient heat removal, and at the same time serve as electrical contacts that allow individual addressing of elements 50 in the VECSEL array 1.

The vertical contact posts 6d extend from the indium solder bumps 5 to the bottom contact 6b

Excitation of the active layer 2 is achieved by sending an electrical current from the top contact 6e through the top DBR 2a and the opening in the oxide layer 2b as well as back through the bottom contact layer 6a, the bottom contact 6b, and the vertical contact post 6d to the metal lines 10, which provide external contacting means. The resultant divergent beams 8 exit through the respective microlens 3.

The VECSEL beams 8 retain a high degree of circular symmetry, with a small divergence angle. The use of a second microlens array 3a, integrated on a second transparent substrate such as glass or sapphire substrate 7a, allows the beams 8 to be collimated/routed as collimated beams 9 for subsequent coupling into optical fibre ports (not shown). As an alternative to the microlens, a diffractive optical element can be used.

Heating is a far more critical issue for VECSEL design because the VECSEL 1 tends to generate considerable heat per mA of current, as the top DBR 2a makes the VECSEL 1 rather resistive. To aid heat sinking in the VECSEL 1 , a heat sink 4 composed of a material of high thermal conductivity such as diamond or copper is placed in close vicinity to the active layer 2 and the top DBR 2a. In order to provide electrical isolation between the elements of the array for individual addressing, the heat sink 4 has to be structured with photolithographically defined metal pads indicated above the indium solder bumps 5. In case of electrically conductive heat sink material such as copper, the pads are placed on top of a thin dielectric layer. In the VECSEL array 1 , crystal heating leads to band-gap shrinkage and therefore a shift in the wavelength of the peak gain. Lattice heating also heats the carriers, spreading out the Fermi-Dirac distributions of carriers in the conduction and valence bands and thus reducing the gain. Together with increasing carrier leakage from the active region at higher temperatures, this effect limits the achievable output power of the VECSEL array 1. Since the cavity length is of the same order of magnitude as that of an edge-emitting laser, the VECSEL array 1 has densely spaced longitudinal modes and thus the emission wavelength will essentially follow the shift of the gain peak.

VECSEL fabrication relies on the same techniques that are successfully employed in conventional VCSEL technology, comprising, among others, semiconductor epitaxial growth preferably by molecular beam epitaxy or metal-organic vapour phase epitaxy, electrical contact deposition, and lateral structuring for current and optical confinement involving photolithographic steps.

The performance of the VECSEL array 1 can be attributed to improvements in its electrical characteristics, reducing the large voltage drops of the DBR 2a, and also the development of the native oxide confinement layer 2b within the laser structure, which provide both electrical and optical transverse confinement. For applications, where arrays of VECSELs may operate simultaneously and in close proximity, it is imperative that the threshold currents and voltages be minimised and made uniform to reduce the thermal dissipation and crosstalk.

The shape of the emitted laser beam 8, also denoted as the transverse mode profile, of each individual element within the VECSEL array 1 on the one hand side depends on the aperture size and the injection current. Most importantly, transverse mode selection is determined by the cavity configuration. Dependent on the mode diameter and thus the amount of diffraction during mode field propagation, the spacing between the top DBR 2a and the microlens array 3, has to be adjusted properly in order to provide for a stable resonant cavity. Adjustment is achieved by the correct choice of thickness of the GaAs substrate 6 and of the substrate 7 supporting the microlens array 3. Diffraction is larger for higher transverse order modes and these excess losses ensure the desired oscillation on the fundamental mode that is ideally suited for efficient single-mode fibre coupling.

The VECSEL array 1 of the present invention allows beams with a high pump power of more than 70 mW to be coupled into erbium-doped fibre array (EDFA) ports (not shown), making them perfect candidates for multi-port optical amplification. In addition, high-level of electronic and optical integration for efficient automatic pump control via software, can be embedded. A driver (not shown) for the VECSEL array 1 and a photodetector array 12, to monitor for the pump powers of the VECSEL array, can be built-in. The photodetector array 12 can be integrated on an ultra-thin semiconductor chip 13. A digital control system can be implemented, comprising a processor (not shown) with an interface and command set. Integration of the VECSEL array 1 is simple with, for instance, standard +5 VDC and GND power lines and a RS-232 interface. The signals from the photodiodes can be supplied to the processor to enable control signals for the driver.

An external cavity resonator can be used in conjunction with a conventional VCSEL structure to provide higher effective active diameter and improved loss discrimination between transverse modes.

The projected features of the VECSEL array according to preferred embodiments of the present invention can be summarised as follows:

- , High port counts: > 15 elements.

- High gain: > 100 mW of pump power per element can be generated.

- Integration with EDFA array: The use of appropriate pump/signal routers results in efficient multi-port optical amplification.

- Pump power control: The power of the individual VECSEL elements can independently be adjusted to support dynamic EDFA gain, configured via simple software commands. Volume production: VECSELs are designed for volume production.

The embodiments have been described by way of example only and modifications are possible within the scope of the invention.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

The Claims Defining the Invention are as Follows

1. A semiconductor laser element comprising a first reflector and a second reflector, and an active region provided between the first and second reflectors for providing optical amplification by stimulated electron-hold recombination, wherein the second reflector is curved to thereby establish a semi-spherical cavity between the first and second reflectors.

2. A semiconductor laser element according to claim 1 , wherein the curved second reflector is a microlens provided with a reflective coating.

3. A semiconductor laser element according to claim 2, further comprising a semiconductor substrate onto which the microlens is mounted.

4. A semiconductor laser element according to claim 3, wherein the microlens is mounted on a first optical substrate attached to the semiconductor substrate.

5. A semiconductor laser element according to any preceding claim, further comprising a relay means provided on a second optical substrate.

6. A semiconductor laser element according to claim 5, wherein the relay means comprises a microlens or diffractive optical element.

7. A semiconductor laser element according to any preceding claim, wherein the first reflector is a distributed Bragg reflector.

8. A semiconductor laser element according to claim 7, wherein the distributed Bragg reflector comprises a multiplicity of pairs of layers of high and low refractive index layers.

9. A semiconductor laser element according to any preceding claim, further comprising a heat sink arranged to dissipate heat generated by current flow through the laser element.

10. A semiconductor laser element according to any preceding claim, wherein the active region comprises multiple quantum wells.

11. A semiconductor laser element according to any preceding claim, further comprising contact means for providing excitation of the active region.

12. A semiconductor laser element according to any preceding claim, further comprising monitoring means for monitoring an optical beam generated within the active region.

13. A semiconductor laser array comprising a plurality of semiconductor laser elements, each of the elements comprising a first reflector and a second reflector, and an active region provided between the first and second reflectors for providing optical amplification by stimulated electron-hold recombination, wherein the second reflector is curved to thereby establish a semi-spherical cavity between the first and second reflectors.

14. A semiconductor laser array according to claim 13, wherein each element further comprises contact means for providing excitation of the active region.

15. A semiconductor laser array according to claim 13 or 14, wherein each element further comprises monitoring means for monitoring an optical beam generated within the active region.

16. A semiconductor laser array according to claim 15, further comprising processing means for controlling the generation of optical beams by individual elements in response to the monitoring means.17. A semiconductor laser array according to claim 15 or 16, wherein the plurality of elements comprise a two-dimensional array and the monitoring means comprise a corresponding two-dimensional photodetector array.

18. A semiconductor laser element substantially as hereinbefore described with reference to Figure 3.

9. A semiconductor laser array substantially as hereinbefore described with reference to Figure 3.