Lasers

Details Lasers Lasers

1999-06-14

2001-06-19

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular active media

Gas

C372S092000, C372S108000

Reexamination Certificate

active

06249536

ABSTRACT:

This invention relates to lasers.
There are many applications for which high power semiconductor lasers are suitable particularly where operation is required in the near infra-red. These include materials processing and micromachining. A requirement in these applications is that the output of the laser can be focussed to a small spot so that a material may be sufficiently heated to cause a chemical change, ablation or burning. Other applications include range finding and as pumps for fibre lasers. In these applications optical brightness is a key parameter and should be as high as possible. The optical brightness, which determines the size of the spot to which the emitted light can be focussed, is given by the power per unit area per unit solid angle.
There is a limit to the brightness of light which can be emitted by a semiconductor laser. This is due to the optical power handling capacity of the surface or facet from which the light is emitted. If the optical power density in this region is too high this can lead to degradation of the laser. Therefore, at the maximum optical power density, the brightness can only be increased by reducing the divergence of the beam from the laser.
A number of methods have been proposed for increasing the brightness of semiconductor lasers. These include fabricating a waveguide comprising a tapered laser (K. A. Williams et al “Q-switched bow-tie lasers for high energy picosecond pulse generation”, Electronic Letters, volume 30, no. 4, pp320-321, 1994), tapered laser amplifiers (D. Mehuys “5.25-W CW near diffraction limited tapered stripe semiconductor optical amplifier”, IEEE Photonic Technical Letters volume 5, no. 10, pp1179-1182, 1993) and anti-guide laser arrays (D. Botez, “High power monolithic phase-locked arrays of anti-guide semiconductor diode lasers”, IEE Proceedings, volume 139, pp14-23, 1992). A disadvantage of tapered laser waveguides and tapered laser amplifiers is that they have a limited power range over which they will maintain a narrow lateral divergence. At powers above this limited range, optical and carrier induced index chances disrupt the waveguide and cause filamentation of emitted light which leads to an increase in the beam divergence. Although anti-guide lasers have been proposed as a solution to this problem, they are difficult to make due to the very tight tolerances required for the waveguide widths. Furthermore, a substantial fraction of the output power is emitted in side lobes, which decreases the brightness of these devices.
According to the invention is provided an incoherent array of tapered semiconductor lasers.
Preferably the tapered laser array comprises a plurality of waveguides. Each waveguide may comprise an individual tapered laser. Preferably each waveguide will support only one transverse optical mode. Alternatively two or more optical modes may be supported. Preferably each waveguide has a narrow region and a tapered region which tapers out to a wide output region.
Preferably the array is formed by depositing layers on a single substrate. Preferably the array is formed on a single laser chip. Waveguides on the chip may be formed by an etching step of one or more layers. All of the tapered lasers may be formed together.
Limiting the number of modes which will be generated in each waveguide increases the brightness, reduces internal losses and increases internal efficiency of the device.
Preferably the lasers are ridge loaded structures. Alternatively they may be buried structures.
Limiting the number of modes may be achieved by having a small refractive index step. Limiting the number of modes may also be achieved by having a narrow width at one end of the waveguides. This may cut off higher order modes. In one embodiment the narrow region of the waveguides may have a width of less than 10 &mgr;m, preferably it is 5 &mgr;m or less and most preferably it is between 3 and 5 &mgr;m. The refractive index step may be less than 0.3. Preferably it is 0.2 or less, most preferably it is between 0.005 and 0.015. Other combinations of width and refractive index step may also provide suitable mode selection. The narrow end may be referred to as the input end because it is at the other end of the waveguides to the output end.
Preferably each waveguide tapers out to a wide output end. The term wide is relative and means that it is wider than the narrow region. The width of each waveguide at the output end may be chosen such that the optical intensity is not sufficient to cause damage to a facet at the output end. Preferably the taper spreads out to a width of between 5 and 100 &mgr;m. Most preferably the width is approximately 30 &mgr;m. The width must be small compared to the taper length so that the resulting taper angle is low enough to prevent coupling to higher order modes.
Each waveguide may comprise a straight section and a tapered section. The sides of the tapered section may be straight or may be shaped in some other manner. They may follow a parabolic shape. A parabolic shaped taper has the advantage of giving lower transverse mode coupling than a taper with straight sides. For a parabolic shape, the sides of the taper may diverge most at a region near the straight section and then diverge less and less along the taper away from the straight section. The sides of the taper may be substantially parallel at the output end.
The width of the beam emitted by the array is dependent on the number of waveguides. Therefore the array may be scaled-up by increasing the number of waveguides whilst still maintaining a low divergence output.
The waveguides may be between 500 and 2000 &mgr;m long. Preferably they are between 700 and 1200 &mgr;m long. Most preferably they are approximately 1000 &mgr;m long. In one embodiment the straight section of each waveguide is between 100 and 300 &mgr;m long and the tapered section is between 600 and 900 &mgr;m long. The length can be set to optimise the external quantum efficiency of the waveguides while maintaining taper angles shallow enough for single mode operation.
Preferably the waveguides are defined in an active layer by a conductive structure of an appropriate shape for example tapered.
An angle defined by the edge of the waveguide and the longitudinal axis of the waveguide is referred to as the taper angle. Preferably the taper angle is less than 2°.
Preferably each waveguide has a reflection coating applied at each end. Preferably a high reflectivity coating is present at the input end. Preferably the reflectivity is more than 95%. Most preferably it is approximately 98%. Preferably a low reflectivity coating is present at the output end. Preferably the reflectivity is less than 3%. Most preferably it is between 0.5 and 1%.
Preferably the waveguides are closely spaced such as to provide a relatively uniform brightness across the width of the output end of the array. Preferably the space between adjacent waveguides at the output end is less than 500 &mgr;m, more preferably less than 200 &mgr;m. Most preferably it is between 5 &mgr;m and 100 &mgr;m. In various preferred embodiments it may be approximately 10, 20, 30, 40, 50, 60, 70, 80 or 90 &mgr;m.
Preferably the lasers operate in the range 0.8-2.5 &mgr;m. Most preferably they operate in the range 0.8-1 &mgr;m. However by choosing the correct material system such as GaN then they may operate as low as 0.4 &mgr;m.
By using an array of tapered lasers, light may be emitted having both a narrow lateral beam divergence and high output power.


REFERENCES:
patent: 4251780 (1981-02-01), Scifres et al.
patent: 4349905 (1982-09-01), Ackley
patent: 4852113 (1989-07-01), Botez
patent: 4942585 (1990-07-01), Ungar
patent: 5321718 (1994-06-01), Waarts et al.
patent: 5537432 (1996-07-01), Mehuys et al.
patent: 5539571 (1996-07-01), Welch et al.
patent: 5544268 (1996-08-01), Bischel et al.
patent: 5602864 (1997-02-01), Welch et al.
patent: 5703897 (1997-12-01), Welch et al.
patent: 5761234 (1998-06-01), Craig et al.
patent: 5793521 (1998-08-01), O'Brien et al.
patent: 0226445 (1987-06-01), None
patent: 0337688 (1989-10-01), None

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