WO2002041456A2

WO2002041456A2 - Laser devices

Info

Publication number: WO2002041456A2
Application number: PCT/GB2001/005033
Authority: WO
Inventors: Claudio Marinelli; Judy Megan Rorison; Laurence Sargent; Richard Vincent Penty; Michal Bordovsky; Ian Hugh White
Original assignee: University Of Bristol
Priority date: 2000-11-15
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: AU2002223807A1; WO2002041456A3; GB0027895D0

Abstract

This invention concerns the use of engineered deep-etch air/semiconductor Bragg reflector gratings in order to control the spectral and spatial emission in short-wavelength laser diodes. The technique consists of introducing a high-order deep-etch distributed reflector of controlled depth, pitch and position into the laser cavity, in order to selectively enhance the modal reflectivity of one, or a selected number, or lasing modes.

Description

Laser Devices-

The present invention relates to laser devices, and in particular, but not exclusively, to nitride based laser devices.

Background of the Invention

Control over the spatial and spectral emission characteristics of nitride-based laser diodes (LDs) is crucial for many applications, such as spectroscopy, gas analysis, optical data storage, laser printing, scanning. In optical data storage and laser printing, a high quality beam profile is required; However, poor optical confinement and subsequent radiation leakage from the waveguiding region into a gallium nitride GaN substrate can give rise to lasing in high-order transverse modes with multi-spot far field patterns. High quality single-spot far field patterns have been achieved by replacing the thick GaN: Si contact layer with an AlGaN:Si layer.

Mode-hop free blue/uv laser diodes, operating in a single longitudinal mode, are desirable for spectroscopy, gas analysis, violet-sensitive photopolymer phase and many other applications. Stable single-frequency emission has been obtained from nitride LDs using external cavities, or long scale distributed feedback (DFB) and distributed Bragg reflector (DBR) techniques. However, these approaches to improving mode control require complicated and expensive fabrication procedures .

Enhancement of the facet reflectivity through the use of high-reflectivity (HR) dielectric coatings is also desirable for obtaining high performance nitride lasers that achieve continuous-wave operation. However, the low refractive index of nitride compounds (~2.5) limits the maximum achievable reflectivity at the nitride/air interfaces to 18%. The manufacturing of high performance GaN LDs therefore requires an additional fabrication step to enable an HR coating to be applied to the laser facets.

Summary of the present invention

In one embodiment of the present invention, it is possible to enhance the spectral and spatial emission characteristics of a short-wavelength (for example 407m) nitride-based laser diode by introducing a 2.5 periods high-order air/semiconductor Bragg grating of controlled depth, pitch and position.

Brief description of the drawings:

Figure 1 is a plan view photograph of a^' device embodying one aspect of the present invention;

Figure 2 is a schematic diagram of a device embodying one aspect of the present invention;

Figure 3 illustrates the dependence on air gap thickness of the modal reflectivity of air/semiconductor Bragg gratings for an ideal Gaussian beam (spot size = 360nm) ;

Figure 4 illustrates the degradation in the calculated modal reflectivity of air/semiconductor Bragg gratings as function of the grating order;

Figure 5 illustrates optical emission spectra of an InGaN MQW laser diode before (top curve) and after (bottom curve) the introduction of an air/semiconductor Bragg grating;

Figure 6 illustrates the calculated near field distribution for the fourth (filled gray curve) and ninth (dotted black curve) order transverse modes;

Figure 7 illustrates the light-current characteristics before (top line) and after (bottom line) etching the Bragg grating; and

Figure 8 illustrates the near filed patterns measured before and after the introduction of an air/nitride Bragg grating.

Detailed Description

Figures 1 and 2 show a laser diode device embodying one aspect of the present invention.

The laser diode device of Figures 3 and 4 is a ridge- waveguide InGaN/GaN multi-quantum well laser diode operating at a centre wavelength of about 407nm. The laser epitaxial layers are grown on a sapphire substrate. The device mesa structure and facets are fabricated by reactive ion etching (RIE) . The results in llOOμm long and 4μm wide Fabry-Perot cavities. The laser facets are uncoated. It will be readily appreciated that this laser diode device is merely one example of a device embodying the present invention. Other laser diodes can embody the invention. Focussed ion beam (FIB) etching can be used to fabricate a deep etch Bragg reflector grating structure. Any suitable etching technique may be used instead of FIB-etching. In the example, three lOμm long trenches are etched so as to be orthogonal to the longitudinal direction of the cavity. The etch defines a 5A/4 Bragg grating consisting of 2.5 air/semiconductor pairs (figure 1) . In the example the grating structure is introduced at one end of the laser cavity and is positioned at a distance of 5μm form the back facet (see figure 2) . The air/nitride mirror is etched using an FIB current of 200pA and an FIB dose of 4000pC/μm². The ion dose controls the etch depth and, due to the Gaussian profile of the etches, the air- gap/nitride-layer thickness ratio. From the SEM

^■ pictures the grati-ng pitch is found to be to be in the range 700-730nm; the ideal pitch value for a 5^th order grating at 407nm is 711nm.

One of the major advantages of using air/nitride Bragg gratings on short wavelength devices is the reduction in diffraction effects in the air gaps in contrast with longer wavelength devices. The light diffraction in the mirror section can severely limit the performance of air/semiconductor Bragg gratings. Figures 3 and 4 compare the calculated dependence of the modal reflectivity of an ideal Gaussian beam on the air gap thickness and the grating order for 407nm and 980nm air/semiconductor Bragg grating lasers. The calculations take into account multiple reflections and light diffraction in the various grating sections. As you can see from Figures 3 and 4, the order and the period of the air/nitride grating can be increased without significant degradation of reflectivity at a wavelength of 407nm. The grating period and the trench width can therefore be increased to a size for which standard lithography manufacturing techniques can be used. This means that for short wavelength devices the etching of mesa structure, facet and high-order air/semiconductor grating can be performed in a single fabrication process. It will be appreciated that "high order" in this context means any DBR having grating spacing larger than λ/4 (a first order grating) for example, a 3^rd order (3λ/4) grating structure is considered to be "high order", as is an eleventh order (llλ/4) grating.

The post-etch performance of devices embodying the present invention has been compared with pre-tech performance, at room temperature under pulsed conditions (200ns pulses at a reception rate of 3kHz) . Figure 5 illustrates the suppression of spectral components arising from the presence of the Bragg grating. While the original Fabry-Perot device always exhibits multi-peak emission (top spectrum) , the etched laser embodying the invention produces a single spectral feature (bottom spectrum) . A reduction in spectral linewidth of up to 63% is observed. Within the limits of the experimental spectral resolution (0.5nm), the introduction of the air/nitride mirrors therefore results in single-mode emission up to currents equal to 1.141th-

Figure 7 shows that a 7% reduction in threshold current (from 570 to 540mA) is possible upon the introduction of the air/nitride Bragg grating. These characteristics are the result of the increased reflectivity and enhanced spatial mode selectivity introduced by the air/nitride grating.

Figure 6 shows the calculated near field distribution for two transverse modes, which in the etched laser structure, exhibit a strong overlap with the active region. The calculation is performed for an AIGaN/GaN/InGaN MQW structure grown. on a sapphire substrate, as described by the refractive index profile in 'figure 3 (topmost curve). The optical confinement factors, r and Tg, for the fourth and ninth order transverse modes are equal to 0.0215 and 0.0031, respectively. Such devices are most likely to emit in the fourth order transverse mode. This suggests that the difference in threshold between the fourth and ninth order transverse modes could be increased by introducing a Bragg mirror structure of controlled depth to enhance selectively the reflectivity for only the fourth order mode. This concept has been exploited experimentally to achieve the results illustrated in figures 5 and 7. It has also been demonstrated that a grating formed by deeper etches, which have been produced using a higher FIB dose (5000pC/μm²) , gives rise to a higher reduction in threshold current (13%) , and results in multi-mode emission. A contribution to modal selectivity also arises due to the inherent wavelength dependence of the grating reflectivity.

Further evidence of the effect of the Bragg grating is • 'shown in the near filed patterns (NFPs) of figure 8. From the side-lobe in the NFP of Fabry-Perot laser it is clear that the device is lasing in a high order transverse mode. The introduction of the air/nitride Bragg grating tends to suppress any side-lobes and broadens the NFP. The broadening is due to enhanced reflectivity across a region wider than the original ridge profile.

Hence, devices embodying the present invention using a high-order air/semi-conductor Bragg grating with a reduced number of periods (2.5) can be used not only to reduce the threshold current of the device but also to enhance the spatial mode selectivity of short- wavelength nitride-based lasers. It is also possible to control the .degree of threshold reduction and mode selectivity by varying the etch depth. The theoretical limitation of the grating performance due to multiple reflections and light diffraction effects in the Bragg mirror grating is significantly reduced in the case of short-wavelength emitter. It has been shown that the grating period can be defined using standard lithography techniques. In another aspect of the invention, a single-step manufacturing process is used to integrate the etching of the mesa structure, the facets and a high-order deep-etch Bragg grating with mode-selective properties.

The calculations and experimental studies described above indicate that the reflectivity of air/semiconductor Bragg gratings at short wavelength is not significantly limited by the multiple reflections and light diffraction effects in the grating sections.

Therefore, the period and order of the Bragg grating can be substantially increased without significant performance degradation, so that standard lithography techniques can be used to define the Bragg reflectors.

This leads to the possibility of integrating the etching of the mesa structure, the facet and the Bragg grating in a single fabrication step.

The two curves of Figure 3 compare the degradation in performance of air/semiconductor Bragg gratings on 407nm nitride-based lasers and 980nm arsenide-based lasers .

Figure 4 illustrates a comparison between the performance of air/semiconductor Bragg gratings on 407nm nitride-based lasers and 980nτn arsenide-based lasers is shown.

Claims

1. An optoelectronic component, comprising a nitride based laser diode structure, and a high-order deep etch grating structure integrated with the laser ^• diode structure.

2. A component as claimed in claim 1, wherein the laser diode structure is a ulti quantum well structure.

3. A component as claimed in claim 1, wherein the laser diode structure is a Fabry-Perot structure.

4. A component as claimed in any one of claims 1 to 3, wherein the high-order grating exhibits low diffraction loss at wavelengths below 700nm.

5. A component as claimed in any one of the preceding claims, wherein the grating structure has properties determined by a wavelength of operation of the component .

6. A component as claimed in claim 5, wherein the depth, order, pitch, overall shape and position of the grating are selected such as to selectively overlap spatially and/or spectrally with at least one, transverse or longitudinal mode.

7. A component as claimed in any one of the preceding claims, wherein the grating structure comprises trenches that extend into the laser diode structure substantially perpendicularly to an upper surface of the laser diode structure.

8. A component as claimed in any one of claims 1 to 6, wherein the grating structure comprises trenches that extend into the laser diode structure at respective predetermined angles to an upper surface of the laser diode structure.

9. A component as claimed in any one of claims 1 to 6, wherein the grating structure is defined by regions of material of refractive index different to that of the material of the laser diode structure.

10. A component as claimed in any one of claims 1 to 8, wherein the grating structure is defined by regions of material of different gain or loss to that of the material of the laser diode structure.

11. A component as claimed in any one of claims 1' to 6 and 10, wherein the grating structure is defined by regions of material of different refractive index to that of the material of the laser diode structure.

12. A component as claimed in claim 1, wherein the grating structure extends a predetermined distance towards an active region of the laser diode structure.

13. A component as claimed .in any one of the preceding claims, wherein the grating structure is arranged in a pumped or an un-pumped region of the laser diode structure.

14. A component as claimed in any one of the preceding claims, comprising a plurality of such grating structures.

15. A component as claimed in anyone of the preceding claims, wherein the laser diode structure is operable to be pumped electrically or optically.

16. A component as claimed in any one of the preceding claims 3, further comprising isolated electrical contacts.

17. A component as claimed in any one of the preceding claims, integrated with at least one of an amplifying section, an absorbing section and a passive section.

18. A component as claimed in claim 17, where the amplifying section or absorbing section has a modulated gain/loss characteristic.

19. A component as claimed in any one of the preceding claims, which component is pulsed by gain switching, Q-switching or mode-locking techniques), when in use.

20. A component as claimed in any one of the preceding claims, which is operable to emit an optional signal in the plane of the device.

21. A component as claimed in any one of claims 1 to 19, which is operable to emit an optical signal out of the plane of the device.

22. A method of manufacturing a nitride-based optical device, the method comprising: providing a substrate; providing a nitride-based laser diode structure on a surface of the substrate; and -defining a high-order deep etch grating in a ^■ surface of the laser diode structure.

23. A method as claimed in claim 22, wherein the grating structure is defined in the laser diode structure using lithography techniques.

24. A method as claimed in claim 22, wherein the grating structure is defined in the laser diode structure using focussed ion beam etching techniques.

25. A method for controlling a mode of an optical signal output from a nitride-based laser diode, the method comprising providing a high-order deep etch grating in the laser diode.