Optically pumped surface emitting lasers incorporating high reflectivity/bandwidth limited reflector
Optically pumped laser structures incorporate reflectors that have high reflectivity and are bandwidth limited to a relatively narrow band around the central laser radiation wavelength. In some cases, the reflectors may be ¾-wavelength distributed Bragg reflectors (DBRs).
1. A laser structure, comprising:
a vertical cavity surface emitting laser structure, comprising:
a GaN substrate;
a group III-Nitride semiconductor gain region disposed on the GaN substrate and configured to emit laser radiation at a central laser wavelength, λ l , having a range between about 370 nm to about 550 nm;
a pump source configured to emit a pump radiation beam at a central pump wavelength having a range between about 350 nm to about 530 nm directed toward the gain region;
a first reflector comprising a distributed Bragg reflector (DBR) disposed in the pump radiation beam, the first reflector having a relatively narrow reflectivity bandwidth for the laser radiation with reflectivity greater than about 90% across a bandwidth of about 60 nm centered at the central laser wavelength, the first reflector also having a reflectivity less than about 50% at the central pump wavelength, the first reflector having alternating layers, the i th layer of the first reflector having a thickness t 1i =3λ l /4 η 1i (λ l ), where η 1i (λ l ) is a refractive index of the ith layer of the first reflector at λ l ; and
a second reflector comprising a DBR, the second reflector having alternating layers, the i th layer of the second reflector having a thickness t 2i =λ l /4 η 2i (λ l ), where η 2i (λ l ) is a refractive index of the i th layer of the second reflector at λ l , the second reflector having a relatively wide reflectivity bandwidth with reflectivity greater than about 90% for both the laser radiation and the pump radiation, wherein the gain region is disposed between the first reflector and the second reflector.
2. The laser structure of claim 1 , wherein the first reflector comprises more than about 6 alternating layer pairs.
3. The laser structure of claim 1 , wherein the first reflector comprises alternating layers of non-epitaxial, dielectric materials.
4. The laser structure of claim 1 , wherein the first reflector comprises alternating layers of epitaxially grown semiconductor materials.
5. The laser structure of claim 1 , wherein the first reflector comprises:
a first portion having a first thermal conductivity; and
a second portion having a second thermal conductivity that is less than the first thermal conductivity.
6. The laser structure of claim 5 , wherein:
the first portion comprises alternating layers of epitaxially grown semiconductor materials; and
the second portion comprises alternating layers of non-epitaxial, dielectric materials.
7. The laser structure of claim 1 , wherein the pump radiation beam is incident on a surface of the first reflector at an angle with respect to normal to the surface of the first reflector.
8. The laser structure of claim 7 , wherein the angle is about 25 to about 30 degrees or about 35 to about 40 degrees and the first reflector has reflectivity less than about 25% at the pump wavelength of about 405 nm.
9. The laser structure of claim 7 , wherein the pump radiation beam is incident on the first reflector at an angle of about 43 degrees and the first reflector has reflectivity less than about 25% at the pump wavelength of about 445 nm.
10. The laser structure of claim 1 , wherein the laser gain region comprises InGaN quantum well layers.
11. A laser structure, comprising:
a vertical surface emitting laser structure, comprising:
a GaN substrate;
a group III-Nitride semiconductor gain region configured to emit laser radiation at a central laser wavelength λ l ;
a pump source configured to emit a pump radiation beam at a central pump wavelength directed toward the gain region;
a heat sink;
a first reflector comprising a distributed Bragg reflector (DBR) disposed proximate the heat sink, the first reflector having a relatively narrow reflectivity bandwidth for the laser radiation, the first reflector comprising:
a first portion proximate to the semiconductor gain region and comprising first layers of a first semiconductor material alternating with second layers of a second semiconductor material, the first layer having an index of refraction η s1 (λ l ) and a thickness t s1 =λ l /4(λ l ) and the second layer having an index of refraction η s2 (λ l ) and a thickness t s2 =λ l /4 η s2 (λ l ); and
a second portion proximate to the heat sink and comprising first layers of a first dielectric material alternating with second layers of a second dielectric material, the first layer having an index of refraction η d1 (λ l ) and a thickness t d1 =3λ l /4 η d1 (λ l ) and the second layer having an index of refraction η d2 (λ l ) and a thickness t d2 =3λ l /4 η d2 (λ l ), the first portion of the first reflector having a thermal conductivity that is greater than a thermal conductivity of the second portion of the first reflector; and
a second reflector having a relatively wide reflectivity bandwidth with reflectivity greater than about 90% for both the laser radiation and the pump radiation, wherein the gain region is disposed between the first reflector and the second reflector, wherein a first portion of the first reflector has a first thermal conductivity and a second portion of the first reflector has a second thermal conductivity that is lower than the thermal conductivity of the first portion, wherein the first portion is closer to the gain region than the second portion.
12. A semiconductor laser structure, comprising:
a vertical surface emitting layer structure, comprising:
a group III-Nitride semiconductor gain region having a first side and a second side and configured to emit radiation at a central laser wavelength;
a pump source configured to emit a pump radiation beam at a central pump wavelength directed toward the gain region;
a first reflector comprising a distributed Bragg reflector (DBR) arranged proximate the first side of the gain region and in the pump radiation beam;
a GaN substrate remnant upon which the semiconductor gain region is disposed, the substrate remnant including a reflecting surface;
a second reflector; and
an air gap between the second reflector and the reflecting surface.
13. The semiconductor laser structure of claim 12 , wherein:
the first reflector having reflectivity greater than about 90% across a bandwidth of less than about 60 nm centered at the central laser wavelength, the first reflector also having a reflectivity less than about 50% at the central peak pump wavelength.
14. The laser structure of claim 12 , further comprising positioning elements configured to adjust a thickness of the air gap.
15. The laser structure of claim 12 , wherein the pump radiation beam comprises multiple modes and the laser radiation comprises a single mode.
16. The laser structure of claim 12 , wherein the second reflector is disposed on a substrate that is substantially transparent to radiation at the central laser wavelength.
17. A method, comprising:
operating a vertical cavity surface emitting laser comprising;
a group III-Nitride semiconductor gain region configured to emit radiation at a central laser wavelength,
first and second reflectors, and
a GaN substrate remnant upon which the semiconductor gain region is disposed, the substrate remnant having a reflecting surface, the first and second reflectors defining a primary laser cavity and the reflecting surface and the second reflector defining a secondary optical cavity;
operating a pump source configured to emit pump radiation having a number of input modes, the pump radiation directed toward and at least partially absorbed in the semiconductor gain region; and
adjusting a gap between the reflecting surface and the second reflector until the laser outputs a number of output modes that is less than the number of input modes.
18. The method of claim 17 , wherein the number of output modes is only one.
19. The method of claim 17 , wherein adjusting the gap comprises increasing or decreasing a length of a secondary cavity until the laser outputs one or more selected output modes.
20. The laser structure of claim 1 , further comprising a substrate disposed between the first reflector and the second reflector.
21. The device of claim 1 , wherein the bandwidth of the second reflector is about 150 nm.
22. The device of claim 11 , wherein the bandwidth of the second reflector is about 150 nm.