WO2008114896A1

WO2008114896A1 - High power single mode optical devices with s-bending ridge waveguide and fabrication method thereof

Info

Publication number: WO2008114896A1
Application number: PCT/KR2007/001498
Authority: WO
Inventors: Si Hyung Cho; Mario Dagenais
Original assignee: Potomac Optronics Inc.
Priority date: 2007-03-16
Filing date: 2007-03-27
Publication date: 2008-09-25

Abstract

Provided are an optical device with an S-bending ridge waveguide including a laser active region having a first index of refraction and laser blocking regions having a second index of refraction that is lower than the first index of refraction, and a fabrication method thereof. The refractive index difference between the laser active region and the laser blocking region is greater than about 0.005. The waveguide width for a gain medium is between 2.5 and 20 μm. The bending angle of the waveguide within an optical cavity is greater than about 2 degree.

Description

HIGH POWER SINGLE MODE OPTICAL DEVICES WITH S-BENDING RIDGE WAVEGUIDE AND FABRICATION METHOD THEREOF

Technical Field

The present invention relates to an optical device, and more particularly, to an optical device such as a semiconductor laser, an optical amplifier, and a broadband light source with a high power single mode S-bending ridge waveguide, and a fabrication method thereof.

Background Art

High power single transverse mode semiconductor lasers, optical amplifiers and broadband light sources are widely used in fiber communication systems such as long-haul, metro and Fiber-To-The-Home Wavelength Division Multiplexing (WDM) optical systems. The number of channels carried by a communication system is directly proportional to the pump power or broadband light source power needed to operate the WDM system. By increasing laser pump power or broadband light source power, fewer lasers are needed to accommodate all channels, likely reducing system cost and complexity.

In medical and cosmetic applications, there are many new consumer applications such as skin care and acne treatment, which requires small compact, efficient and relatively high power single transverse mode laser.

Laser and broadband light source power may be increased by increasing a semiconductor chip length. As the semiconductor chip length increases, gain volume increases thereby increasing power. However, an increased semiconductor chip length may cause an internal power loss of an optical waveguide of a laser, result in a less efficient device. In addition, in order to increase laser and broadband light source power, laser pump power may also be increased by widening an active region of the laser. Resistance decreases as the width of the active region increases, thereby increasing power.

However, an increased active region width may result in inclusion of an unwanted second transverse mode. Such multi-transverse laser mode may adversely affect the performance of fiber communication systems, and are therefore undesirable for such applications.

In order to solve a multi-transverse mode from the widened active region, there are several proposals such as a buried hetero structure with lower index step between laser active and blocking regions (see US Patent No. 6,552,358 by Cho et al), and a buried ridge structure with lower index step between laser active and blocking regions (see US Patent No. 6,635,502 by Cho et al). However, these buried type structures with low index step require costly re-growth step with high technical difficulties in growth. Another possibility with single epitaxy growth is to use a Master

Oscillator Power Amplifier (MOPA) structure, but this introduces complexity in fabrication and difficulty in coupling pump light into a single mode fiber (see, e.g., Cho, et al, "1.9-W Quasi-CW from a Near-Diffraction-Limited 1.55-micron InGaAsP-InP Tapered Laser," IEEE Photonics Technology Letters, vol. 10, No. 8, pp.1091 -1093 August 1998).

Therefore, a need exists for optical devices such as a high power, single mode semiconductor laser, an optical amplifier and a broadband light source that are relatively simple to manufacture with a single growth step, and are compatible with a fiber amplification communication system, cosmetic and medical applications.

Disclosure of the Invention

The objective of the present invention is to provide an optical device with a high power single mode S-bending ridge waveguide that is relatively simple to manufacture with a single growth step, and are compatible with a fiber amplification communication system, cosmetic and medical applications, and a fabrication method thereof.

To achieve the above objective, the present invention provides a high power optical device such as a semiconductor laser, an optical amplifier, and a broadband light source having a single transverse mode operation. In an exemplary embodiment of the present invention, optical power higher than that generated by conventional high power lasers is achieved by widening a gain medium without inducing a second transverse mode. This is accomplished by bending a waveguide within an optical cavity to create more bending loss for a higher order mode while keeping a very low bending loss for a fundamental mode. A refractive index difference between a laser active region and a laser blocking region material may be larger than about 0.005. A waveguide width for the gain medium may be between 2.5 μm and 20 μm. The bending angle of the waveguide within the optical cavity may be greater than about 2 degree.

Brief Description of the Drawings

FIG. 1 illustrates an optical device with a typical straight ridge waveguide; FIG. 1A is a cross-sectional view of the straight ridge waveguide, and FIG. 1B is a top view of the straight ridge waveguide.

FIG. 2 is a top view illustrating a single S-bending ridge waveguide that is appropriate for a semiconductor laser as an optical device according to an embodiment of the present invention. FIG. 3 is a top view illustrating a multiple S-bending ridge waveguide that is appropriate for a semiconductor laser as an optical device according to an embodiment of the present invention.

FIG. 4 is a top view illustrating a single S-bending ridge waveguide that is appropriate for an optical amplifier and a broadband light source as an optical device according to another embodiment of the present invention.

FIG. 5 is a top view illustrating a multiple S-bending ridge waveguide that is appropriate for an optical amplifier and a broadband light source as an optical device according to another embodiment of the present invention.

FIG. 6 illustrates an example of a fundamental mode (Oth order mode) transmission characteristic over 2mm long S-bending ridge waveguide.

FIG. 7 illustrates an example of a higher order mode (1st, 2nd, 3rd and higher modes) transmission characteristic over 2mm long S-bending ridge waveguide.

FIGS. 8 through 11 illustrate a method of fabricating an optical device with a high output single mode S-bending ridge waveguide according to an embodiment of the present invention.

Best mode for carrying out the Invention

The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Embodiments and drawings in the present specification are just illustrative and the invention should not be construed as being limited to the embodiments set forth herein.

FIG. 1 shows a typical ridge waveguide type structure from single step growth quantum well, quantum dot, quantum cascade or bulk type heterostructure laser epitaxy materials. The structure contains a laser active region A and laser blocking regions B that are adjacent to the laser active region A and extend laterally from opposite sides of the laser active region A.

That is, the structure includes a substrate 1 , an active layer 2, and a dielectric layer 3. The active layer 2 is formed on a first conductivity type substrate 1 by single growth step. A lower region of the active layer 2 is formed of a first conductivity type material, an upper region thereof is formed of a material of a second conductivity type which is a conductivity type opposite to the first conductive type, and the upper region is protruded. Here, the first conductivity type may be an N-type and the second conductivity type may be a P-type or an opposite case thereto is possible. The dielectric layer 3 includes an outer circumference of the protruded portion of the upper region of the active region 2 excluding a middle portion of a top surface of the protruded portion and covers the entire top surface of the active region 2.

The active region A is pumped by current, and the blocking regions B are electrically isolated by dielectric materials. The blocking regions B can be formed by etching the active layer 2 from a top surface of a laser epitaxy material, and depositing dielectric materials 3 thereon.

The refractive index of the active region A differs from that of the blocking regions B providing lateral optical confinement. In conventional optical devices with straight ridge waveguides, the effective refractive index difference between the fundamental transverse mode of the active region A and the block regions B is about 0.013. This provides a single transverse mode with an active region stripe width of 2.4 μm.

When the active region stripe width is increased, an unwanted second transverse mode may arise. For example, when the stripe width is increased to 8.0 μm, there are at least four transverse electrical modes with the refractive index step of 0.013. When the refractive index step is decreased, a single transverse mode may be formed from a larger ridge width, but due to low optical confinement and current spreading (nature of ridge waveguide structure), the higher order mode can be easily excited while increasing the operating current of the laser (optical device).

To achieve a single transverse mode with a stripe width larger than 2.5 μm and the refractive index difference larger than 0.005, the S-bending type waveguide can be designed to increase a bending propagation loss for a higher order transverse mode as shown in FIG. 7 while keeping a low bending propagation loss for a fundamental transverse mode as shown in FIG. 6. Here, FIG. 6 illustrates an example of a fundamental mode (Oth order mode) transmission characteristic over 2mm long S-bending ridge waveguide, and FIG. 7 illustrates an example of a higher order mode (1st, 2nd, 3rd and higher modes) transmission characteristic over 2mm long S-bending ridge waveguide.

Embodiments of the above-described optical device (laser) design are shown in FIGS. 2 through 5.

FIG. 2 is a top view illustrating a single S-bending ridge waveguide that is appropriate for a semiconductor laser as an optical device according to an embodiment of the present invention, which illustrates an

S-bending laser waveguide structure having single bending within two laser facets that are aligned to a waveguide with 90 degree.

FIG. 3 is a top view illustrating a multiple S-bending ridge waveguide that is appropriate for a semiconductor laser as an optical device according to an embodiment of the present invention, which illustrates an S-bending laser waveguide structure having a plurality of bending within two laser facets, aligned to a waveguide with 90 degree.

In the embodiments shown in FIGS. 2 and 3, in order to increase a bending propagation loss for higher transverse order modes, a bending angle θ may be larger than 2 degree.

Similar concept can be used for an optical amplifier and a broadband light source to obtain high single transverse mode output power.

FIG. 4 is a top view illustrating a single S-bending ridge waveguide that is appropriate for an optical amplifier and a broadband light source as an optical device according to another embodiment of the present invention, which illustrates possible an S-bend amplifier and broadband light source waveguide structure with single bending within two amplifier facets, aligned to a waveguide with a tilted angle.

FIG. 5 is a top view illustrating a multiple S-bending ridge waveguide that is appropriate for an optical amplifier and a broadband light source as an optical device according to another embodiment of the present invention, which illustrates possible an S-bend amplifier and broadband light source waveguide structure with a plurality of bending within two amplifier facets, aligned to a waveguide with a tilted angle.

In the embodiments shown in FIGS. 4 and 5, this tilted angle design with respect to the both facets is typically used for an optical amplifier and a broadband light source to minimize laser cavity feedbacks.

The active region width of the laser is greater than about 2.4 μm and may be about 2.5 μm to 20 μm. In a particular embodiment, the laser has a refractive index difference between the active and blocking regions of about 0.01 to 0.02 and an active region width in the range of about 3 μm to 8 μm.

Embodiments of the invention provide front facet power greater than or equal to about 100 mW with an illustrative range of 100 mW to 500 mW. A particular embodiment of the invention provides a laser with an active region having a width in the range of about 8 μm to 20 μm and having front facet power in the range of about 500 mW to 1000 mW.

Embodiments of the invention may be applied to lasers of any wavelength but present technology will likely encourage use with lasers having wavelengths of 1480 nm, 1550 nm, 808nm, 1060nm and 980 nm, for example.

A method of fabricating an optical device with a high output single mode S-bending ridge waveguide according to an embodiment of the present invention will now be described with reference to FIGS. 8 through 11. Here, the optical device includes a semiconductor laser, an optical amplifier, and a broadband light source or the like, for example.

First, a substrate 10 formed of material, for example, InP or GaAs is prepared. The substrate 10 has a thickness in the range of about 200 to 500 μm and may have a thickness of about 300 μm. An active layer 20 of a double heterostructure (quantum well and bulk type) may be grown using a variety of epitaxial techniques such as liquid phase epitaxy, hybrid vapor phase epitaxy, metal organic chemical vapor phase epitaxy and molecular beam epitaxy (see FIG. 8A). The active layer 20 of the double heterostructure may have a thickness in the range of about 3 μm to θ μm.

Here, when the substrate 10 is a first conductivity type, the active layer 20 of the double heterostructure is grown by doping a lower region 22 of the active region 20 with a first conductivity type material and by doping an upper region 24 of the active region 20 with a second conductivity type material which is a conductivity type opposite to the first conductivity type. The first conductivity type is an N-type and the second conductivity type may be a P-type or an opposite case thereto is possible.

A photoresist is deposited on the grown active layer 20 of the double heterostructure, and the photoresist disposed on a portion of the active layer 20 which will be removed by etching is patterned using a conventional photolithography technique (see FIG. 8B).

Here, the upper region of the active layer 20 in which the photoresist remains is a region which acts as an S-bending ridge waveguide, and the width of the remaining photoresist 30 corresponds to the width ΔW of the ridge waveguide. Thus, the pattern of the photoresist may be selected according to the types of optical devices, that is, a semiconductor laser, an optical amplifier, and a broadband light source or the like, as illustrated with reference to FIGS. 2 through 5. After that, a portion which will be a blocking region in the upper region 24 of the active layer 20 is etched using chemical solution etching or reactive ion etching (RIE) (see FIG. 8C) by using the remaining photoresist 30 as a mask. The etched depth (ΔH) of the blocking region defines the index step between active and blocking regions. Then, the remaining photoresist 30 is removed, a cross-section thereof is shown in FIG. 9A, and a top surface thereof is shown in FIG. 9B.

After that, dielectric materials are deposited on the entire surface of the resultant structure by chemical vapor deposition, for example, thereby forming a dielectric layer 40 (see FIG. 10B).

Subsequently, the dielectric layer of the portion which will be the active region is etched by chemical solution or reactive ion etching (RIE) by using a conventional lithography step, thereby forming a metal contact opening (see FIG. 10B). After a lower portion of the substrate 10 is removed using a conventional polishing process, for example, so that the thickness of the active layer 20 and the substrate 22 disposed below the dielectric layer 40 is about 100 μm to 150 μm, electrode layers 50 and 60 are formed on a top surface of the upper layer of the active region and a bottom surface of the substrate by depositing proper metals on N- and P-contacts (see FIG. 10C.

The resultant structure may be divided into several parts and separated from a plurality of optical device (laser) chips, and the separated optical device (laser) chips may be bonded to copper or ceramic sinks. Asymmetric high reflectivity coating 70 and anti-reflection mirror coatings 80 are applied to the facets by electron beam evaporation to increase the output power from one side of the laser facets (see FIG. 11 ). As such, low output power is obtained from a side of the high reflectivity coating 70 and high output power is obtained from the anti-reflection mirror coating 80. Embodiments of the invention are particularly applicable to fiber amplifiers, for example, erbium doped fiber amplifiers and Raman fiber amplifiers. Optical devices such as a high power single transverse mode laser, an optical amplifier, and broadband light source according to the present invention can be used in various cosmetic and medical applications.

The exemplary embodiments having been described in detail, many variations and modifications will become apparent to those skilled in the art. Accordingly, it is intended that the invention not be limited to the specific illustrative embodiments but be interpreted within the full spirit and scope of the appended claims.

Industrial Applicability

As described above, according to the present invention, an optical device with a high power single mode S-bending ridge waveguide that is relatively simple to manufacture with a single growth step, and are compatible with a fiber amplification communication system, cosmetic and medical applications can be provided.

Claims

What is claimed is:

1. A method of fabricating a semiconductor optical device comprising: forming a quantum well, a quantum dot, a quantum cascade or a bulk type optical device with single growth step epitaxy layers; forming an active region including an active layer having a first index of refraction by single growth step epitaxy layers; forming a blocking region having a second index of refraction that is lower than the first index of refraction by etching certain depth of materials from upper portions of the single growth step epitaxy layers; forming a wide active region which supports a higher order transverse mode; forming a S-bending waveguide design to introduce a high propagation waveguide loss for a higher order transverse mode while maintaining a very low propagation waveguide loss for a fundamental transverse mode; and forming facets at the end of an optical cavity.

2. The method of claim 1 , wherein the semiconductor optical device is a semiconductor laser, and the forming of facets comprises forming two perpendicular facets at the end of the optical cavity.

3. The method of claim 1 , wherein the semiconductor optical device is an optical amplifier, and the forming of facets comprises forming one or two angled facets at the end of the optical cavity.

4. The method of claim 1 , wherein the semiconductor optical device is a broadband light source, and the forming of facets comprises forming two perpendicular facets at the end of the optical cavity.

5. The method of one of claims 1 through 4, wherein a refractive index difference between the active region and the blocking region is greater than about 0.005.

6. The method of one of claims 1 through 4, wherein the active region and the blocking region are both formed by single growth step epitaxy layers, and the blocking region is introduced by etching certain depth of epitaxy layers from the top surface, and the refractive index of the active region is greater than the refractive index of the blocking region.

7. The method of one of claims 1 through 4, wherein the active region has a width greater than about 2.4 μm.

8. The method of claim 7, wherein of the width of the active region is in the range of about 2.5 μm to 20 μm.

9. The method of one of claims 1 through 4, wherein a bending angle of the waveguide within the optical cavity is grater than about 2 degree.

10. A fiber amplifier and Raman fiber amplifier comprising a semiconductor pump laser according to claim 1.

11. A Raman fiber amplifier comprising a semiconductor pump laser according to claim 1.

12. A skin care and medical laser source comprising a semiconductor laser and a broadband diode light source according to claim 1.

13 A semiconductor optical device including a laser active region and laser blocking regions that are adjacent to the active region and extend laterally from opposite sides of the active region, the optical device comprising: a first conductivity type base layer; a quantum well and bulk type active layer formed on the base layer using a material having a first index of refraction by a single epitaxial growth step, a lower region of the active layer being doped with a first conductivity type material, an upper region of the active layer being doped with a second conductivity type material which is a conductivity type opposite to the first conductivity type, and the upper region having a protruded portion; and a dielectric layer covering the entire top surface of the active layer excluding a top surface of the protruded portion of the upper region of the active layer and having a second index of refraction that is lower than the first index of refraction, wherein the active region protruded portion of the active layer constitutes an S-bending waveguide as the active region and the dielectric layer constitutes the blocking region.

14. The semiconductor optical device of claim 13, wherein the semiconductor optical device is a semiconductor laser and two perpendicular facets are further formed at the end of an optical cavity comprised of the S-bending waveguide.

15. The semiconductor optical device claim 13, wherein the semiconductor optical device is an optical amplifier, and one or two angled facets are further formed at the end of the optical cavity comprised of the S-bending waveguide.

16. The semiconductor optical device of claim 13, wherein the semiconductor optical device is a broadband light source, and one or two angled facets are further formed at the end of the optical cavity comprised of the S-bending waveguide.

17. The semiconductor optical device of one of claims 13 through 16, wherein a refractive index difference between the active region and the blocking region is greater than about 0.005.

18. The semiconductor optical device of one of claims 13 through 16, wherein the active region has a width greater than about 2.4 μm.

19. The semiconductor optical device of claim 18, wherein the active region has a width in the range of about 2.5 μm to 20 μm.

20. The semiconductor optical device of one of claims 13 through 16, wherein a bending angle of the S-bending waveguide is grater than about 2 degree.