Surface emitting laser incorporating third reflector

• US 9,112,331 B2
• Filed: 03/22/2012
• Issued: 08/18/2015
• Est. Priority Date: 03/22/2012
• Status: Active Grant

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Palo Alto Research Center, Inc. (Xerox Holdings Corp.)
• Inventors: Northrup, John E., Wunderer, Thomas, Johnson, Noble M.
• Primary Examiner(s): Niu, Xinning
• Assistant Examiner(s): Vu, Vu A
• Application Number:
  US13/427,105
• Publication Number:
  US 20130343420A1
• Time in Patent Office: 1,244 Days
• Field of Search: 372/50.11
• US Class Current: 1/1
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: H01S 3/109 Frequency multiplication, e.g. harmonic generation
• CPC Class Codes: H01S 5/041 Optical pumping
• CPC Class Codes: H01S 5/141 using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence
• CPC Class Codes: H01S 5/18361 Structure of the reflectors, e.g. hybrid mirrors
• CPC Class Codes: H01S 5/18366 Membrane DBR, i.e. a movable DBR on top of the VCSEL
• CPC Class Codes: H01S 5/18369 based on dielectric materials
• CPC Class Codes: H01S 5/18377 comprising layers of different kind of materials, e.g. combinations of semiconducting with dielectric or metallic layers
• CPC Class Codes: H01S 5/18383 with periodic active regions at nodes or maxima of light intensity
• CPC Class Codes: H01S 5/3201 incorporating bulkstrain effects, e.g. strain compensation, strain related to polarisation
• CPC Class Codes: H01S 5/34333 with a well layer based on Ga(In)N or Ga(In)P, e.g. blue laser

Abstract

Surface emitting laser structures that include a partially reflecting element disposed in the laser optical cavity are disclosed. A vertical external cavity surface emitting laser (VECSEL) structure includes a pump source configured to emit radiation at a pump wavelength, λ_pump, an external out-coupling reflector, a distributed Bragg reflector (DBR,) and an active region arranged between the DBR and the out-coupling reflector. The active region is configured to emit radiation at a lasing wavelength, λ_lase. The VECSEL structure also includes partially reflecting element (PRE) arranged between the gain element and the external out-coupling reflector. The PRE has reflectivity of between about 30% and about 70% for the radiation at the lasing wavelength and reflectivity of between about 30% and about 70% for the radiation at the pump wavelength.

Claims

1. A vertical external cavity surface emitting laser (VECSEL) structure, comprising:

an external out-coupling reflector;

a distributed Bragg reflector (DBR); and

a III-N heterostructure epitaxially grown on a GaN substrate and comprising;

an active region arranged between the DBR and the out-coupling reflector, the active region configured to emit radiation at a lasing wavelength, λ

_lase in response to pump radiation emitted from a pump source and having pump wavelength, λ

_pump; and

a partially reflecting element (PRE) comprising alternating epitaxial layer pairs, including a first layer of AlGaN, InAlN, or InAlGaN and a second layer of GaN or InAlGaN, the partially reflective element arranged between the active region and the external out-coupling reflector, the partially reflecting element having reflectivity of between about 30% and about 70% for radiation at the lasing wavelength and reflectivity of between about 30% and about 70% for radiation at the pump wavelength.

2. The structure of claim 1, wherein the PRE has reflectivity of between about 40% and about 60% for radiation at the lasing wavelength and reflectivity of between about 40% and about 60% for radiation at the pump wavelength.

3. The structure of claim 1 wherein, during operation of the VECSEL structure, the PRE is configured to provide a peak E² field in the active region that is greater than the average E² field in the substrate by a factor of more than 2.

4. The structure of claim 1 wherein, during operation of the VECSEL structure, the PRE is configured to provide a peak E² field in the active region that is greater than the average E² field in the substrate by a factor of more than 3.

5. The structure of claim 1, wherein the radiation emitted by the pump source is incident on a surface of the substrate at an angle θ

such that sin(θ

)=n_sub sin[cos<sup>−

1</sup>(λ

_pump/λ

_lase)] where the index of refraction of the substrate is n_sub.

6. The structure of claim 5, wherein the external out-coupling mirror, DBR, active region, and PRE are arranged so that more than about 50% of the pump radiation is absorbed in the active region.

7. The structure of claim 5, wherein the external out-coupling mirror, DBR, active region, and PRE are arranged so that more than about 75% of the pump radiation is absorbed in the active region.

8. The structure of claim 1, wherein the number of layer pairs is between about 2 and 20.

9. The structure of claim 1, wherein the number of layer pairs is between about 10 and 12.

10. The structure of claim 1, wherein a thickness of the first layer is about 50 nm and a thickness of the second layer is about 46 nm.

11. The structure of claim 1, wherein the first layer is AlGaN and the second layer is GaN and a thickness of the first layer is about λ

_lase/4n_AlGaN(λ

_lase) and a thickness of the second layer is about λ

_lase/4n_GaN(λ

_lase).

12. The structure of claim 1, wherein an antireflective coating is disposed between the substrate and the out-coupling reflector.

13. The structure of claim 12, wherein the antireflective coating has an index of refraction n_AR and a thickness t_AR such that t_AR=(λ

_pump/4n_AR) cos [sin<sup>−

1</sup>((1/n_AR)sin θ

)].

14. The structure of claim 1, wherein the active region comprises:

one or more quantum well structures, each quantum well structure comprising;

one or more In_xGa_1-xN quantum wells, where 0.10≦

x≦

0.5, each quantum well having a thickness, W, and configured to emit laser radiation having a wavelength λ

_lase.

15. The structure of claim 14, wherein:

each quantum well structure includes two or more quantum wells with a thin GaN spacer layer having a thickness, T, disposed between each pair of quantum wells; and

each active region element includes two or more quantum well structures with a thick GaN spacer layer having a thickness, L, where L is greater than T, disposed between each pair of quantum well structures.

16. The structure of claim 15, further comprising a GaN end spacer layer having a thickness IS, the GaN end spacer layer disposed between the PRE and one of the quantum well structures, wherein the thicknesses W, T, L, and IS are configured so that antinodes of a standing wave of the laser radiation overlap the quantum wells of the quantum well structures.

17. A surface emitting laser structure, comprising:

a first reflector;

a second reflector;

an active region comprising multiple active region elements disposed between the first reflector and the second reflector, each active region element, comprising;

one or more quantum well structures comprising one or more In_xGa_1-xN quantum wells disposed between barrier layers, where 0.10≦

x≦

0.5, the quantum wells configured to emit radiation having a wavelength, λ

_lase; and

a pre-strain layer comprising InGaN; and

a GaN end spacer layer disposed between the second reflector and one of the quantum well structures, wherein the active region and the end spacer are configured so that the antinodes of the radiation overlap quantum well structures of the active region.

18. The structure of claim 17, wherein each active region element includes an AlGaN confinement and strain management layer.

19. The structure of claim 18, wherein:

the one or more In_xGa_1-xN quantum wells comprise at least two In_xGa_1-xN quantum wells and a thin GaN spacer having thickness, T, is disposed between each pair of quantum wells in each quantum well structure; and

each active region element comprises at least two quantum well structures and one or more thick GaN spacers, having thickness, L, wherein one thick GaN spacer is disposed between each pair of quantum well structures, wherein T<

L.