Field modulated vertical cavity surface-emitting laser with internal optical pumping

Abstract: A vertical cavity surface emitting laser diode (VCSEL), includes top and bottom electrically non-conductive mirrors defining a first laser cavity containing a laser mediuru and designed to resonate at a first wavelength. A second laser cavity is designed to resonate at a second wavelength less than the first wavelength. The second laser cavity is optically coupled to said first laser cavity so as to pump said laser medium and create laser action in said first laser cavity. The second laser cavity is normally provided by a VCSEL stacked below the first laser cavity.

Description: This invention relates to the field of laser diodes, and more particularly to a field modulated vertical cavity surface-emitting laser with an internal optical pumping. VCSELs or vertical cavity surface-emitting laser diodes are known in the art. While 850 nm current modulated VCSEL laser diodes are commercially available, it is hard to realize a 1300 nm VCSEL due to technical difficulties. In a VCSEL, the optical cavity needs to be defined by mirrors having high reflectivity and low loss to create laser action at room temperature. In addition, the mirrors need to be electrically conductive in order to allow current injection. Up to the present time, it has not been possible to grow mirrors of the desired quality on substrates made of indium phosphide, emitting at 1300 nm. It has been demonstrated that it is possible to fuse mirrors made of aluminum gallium arsenide and its alloys onto an active layer of if indium gallium arsenide phosphide, emitting at 1300 nm. However, this procedure is complicated. The active layer is only about 100 nm thick and it needs to be separated from the substrate on which it was grown and transported to a substrate containing the first mirror, then the stack consisting of the active layer and the first mirror again need to be transported to a substrate containing the second mirror. Obviously, no significant yield can be expected using this method. Also, three wafers are consumed for each functional device fabricated. It has been proposed that one of the mirrors be replaced by a dielectric mirror using silicon and low refractive index materials such as oxides, nitrides or even an airgap. The problem is that the mirror is then no longer conductive so that carrier injection pumping is not possible. US patent no. 5,513,204 discloses an optically pumped VCSEL structure in which the optically pumped VCSELs are attached to the top side of a shorter wavelength VCSEL serving as the optical pump. Such a structure does not lend itself to high yield, efficient production. An object of the invention is to overcome this problem. According to the present invention there is provided a vertical cavity surface emitting laser (VCSEL) structure, comprising top and bottom mirrors defining a first laser cavity containing a laser medium and designed to resonate at a first wavelength, at least one of said mirrors being electrically non-conductive; and a second laser cavity designed to resonate at a second wavelength less than said first wavelength, said second laser cavity being optically coupled to said first laser cavity so as to optically pump said laser medium and create laser action in said first laser cavity. The first wavelength is about 1300nm and said structure is an integral structure grown without fusion of the layers. The problems of the prior art are overcome by optically pumping the VCSEL operating at the longer wavelength with a VCSEL operating at a shorter wavelength that can be pumped by other means, such as carrier injection. The optical pumping is preferably carried out with the aid of a second VCSEL, which since it operates at a shorter wavelength can be fabricated on the same substrate in a conventional manner using conductive mirrors and carrier injection pumping. Typically, the main VCSEL operates at 1300 nm and the VCSEL that provides the optical pumping operates at 980 nm. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which the single figure is a cross section through a VCSEL in accordance with the invention. In the figure, the 1300 nm VCSEL made in accordance with the principles of the invention comprises a conventional 980 nm VCSEL 1 acting as an optical pump and having a bottom mirror 2 of a GaAs/AlGaAs quarterwavelength stack for 1300nm reflection, an active medium 3 of InGaAs quantum wells confined between GaAs layers or an AlGaAs alloy, and a top mirror 4 of a GaAs/AlGaAs quarterwavelength stack. A backside contact 5 is formed on the backside of the VCSEL 1 and isolation layers 6 are provided in a conventional manner. An intermediate GaAs buffer layer 7 is deposited on the top mirror 4. The bottom mirror 8 of the main VCSEL 9 is grown on the buffer layer 7. The bottom mirror consists of gallium arsenide or an alloy thereof. An active layer 10 consists of indium gallium arsenide phosphide dots grown lattice mismatched to the material of the bottom mirror 8. The quantum dots are embedded in aluminium gallium arsenide or GaAs to form a quantum well. The top mirror 11 consists of a stack of dielectric material such as a silicon dioxide or silicon nitride based stack, or an airgap based stack. A ring-shaped contact 12 provides a field electrode. The mirror 11 is surrounded by a ground contact 13. The above structure can be grown on a single substrate using conventional techniques, i.e. MOVPE, metal organic vapor phase epitaxy or MBE molecular beam epitaxy. The 980 nm VCSEL diode 1 formed in the same structure as the 1300 nm VCSEL 9 acts as an optical pump to create a population inversion the active layer 10 of the main VCSEL 9. Since the pumping VCSEL is part of the same structure, there is no need to provide any optical elements to achieve good optical coupling. The pump VCSEL 1, operating at 980 nm is activated by applying a DC bias between its bottom contact 5 and the bottom mirror 8 of the main VCSEL 9 operating at 1300 nm. The main VCSEL 9 is modulated by using an external electrical field applied perpendicular to the active layer 10, employing the Stark-effect to deliberately change bandgap of the active layer 10 and hence move the emission wavelength into and out of resonance with the optical cavity formed between the top and bottom mirrors. The optical output will therefore be modulated by the electrical field and not as by injected carriers. The advantage of this arrangement is that the 1300 nm VCSEL will always be above threshold current so there is less need to use a monitoring device. The 1300 nm VCSEL 9 has a bottom mirror consisting of aluminum gallium arsenide and its alloys. The active layer consists of indium gallium arsenide phosphide quantum dots which may be grown lattice mismatched to the bottom mirror material. The quantum dots are embedded in aluminum gallium arsenide or GaAs to form a quanturn well. The top mirror will consist of a stack of dielectric material, or possibly air gaps. The electrical field is supplied by contacts 12 on the top surface and a contact 13 to the 1300 nm VCSEL bottom mirror, which may serve as ground. The complete structure described above may be grown in one run and on one substrate and there is no need to fuse any of the semiconductor layers. Several variations of the invention are possible. The invention may apply to other semiconducting material systems, the mirrors may both be dielectric, and in such a case the pump VCSEL top mirror may serve as ground. It may be possible not to use quantum dots for the active layer if in the future homogenous 1300 nm active layers can be grown lattice matched to gallium arsenide. Another variation is to grow or to fuse the 980 nm pump VCSEL onto an indium phosphide substrate containing the 1300 nm bottom mirror and the active layers. The invention meets the need for a 1300 nm light emitting device that may compete successfully traditional 1300 nm edge-emitting laser diodes that are notoriously expensive. The invention is applicable to other semiconducting material systems. The pump VCSEL top mirror may serve as ground. The active layer can also consist of 1300nm strained InGaAs quantum wells lattice matched to GaAs or AlGaAs.

Claims: A vertical cavity surface emitting laser (VCSEL) structure, comprising top and bottom mirrors (8, 11) defining a first laser cavity containing a laser medium and designed to resonate at a first wavelength, wherein said first wavelength is about 1300nm, at least one of said mirrors (11) being electrically non-conductive; and a second laser cavity designed to resonate at a second wavelength less than said first wavelength, said second laser cavity being optically coupled to said first laser cavity so as to optically pump said laser medium and create laser action in said first laser cavity, characterized in that said structure is an integral structure grown on one substrate without fusion of the layers. A structure as claimed in claim 1, wherein the second laser cavity is provided by a VCSEL (1) stacked below said first laser cavity and having conductive mirrors (2, 4) to permit pumping by carrier injection. A structure as claimed in claim 2 , wherein the top mirror (11) of the first laser cavity is made of a dielectric material. A structure as claimed in claim 3, wherein said dielectric material is silicon dioxide or silicon nitride. A structure as claimed in claim 3, wherein the top mirror (11) is made of an airgap based stack. A structure as claimed in any one of claims 1 to 5, wherein said bottom mirror (8) is made from gallium arsenide or an alloy thereof. A structure as claimed in any one of claims 1 to 6, wherein said laser medium consists of indium gallium arsenide phosphide quantum dots or an alloy combination thereof embedded in GaAlAs or GaAs that emits at 1300nm. A structure as claimed in claim 7, wherein said quantum dots are grown lattice mismatched to the material forming the bottom mirror (8). A structure as claimed in any one of claims 1 to 8, further comprising contacts (12, 13) for applying an external electric field to the laser medium to change the bandgap. A structure as claimed in claim 9, wherein said contacts (12, 13) are arranged to apply said field perpendicular to the laser medium. A structure as claimed in claim 10, wherein one of said contacts is located on the top mirror, and the other of said contacts is located laterally adjacent the active medium. A structure as claimed in claim 1 wherein both mirrors (8. 11) are dielectric, A structure as claimed in claim 1, wherein the laser medium comprises 1300nm InGaAsP quantum wells lattice matched to GaAs or AlGaAs. A structure as claimed in claim 1, wherein the top mirror is a dielectric.

Inventors: KULLANDER-SJOBERG ANNA, WIDMAN MICHAEL, KULLANDER-SJOBERG, ANNA, WIDMAN, MICHAEL
Applicants: MITEL SEMICONDUCTOR AB
Application year: 1998
Main classification: H01S5
IPC codes: H01S5, H01S3