KR100553953B1 - Light polarization method of laser devices - Google Patents

Abstract

The present invention relates to a method of optical polarization of a vertical cavity surface emitting laser device such as a vertical cavity surface emitting laser (VCSEL) diode. The present invention comprises the steps of arranging a substrate on which the electrical contact is disposed; Disposing a lower mirror structure on the substrate; Disposing an active region on the lower mirror structure; Disposing an upper mirror structure by the active region; And disposing a contact area on the upper mirror structure to discontinuously form the inside of the body so that the polarization direction of the emitted light is substantially aligned with the boundary line of the discontinuous line.

Laser, discontinuity, optical polarization

Description

LIGHT POLARIZATION METHOD OF LASER DEVICES

1 and 2 are plan and cross-sectional views of a conventional VCSEL diode.

3 and 4 are a plan view and a cross-sectional view of a VCSEL laser device according to a preferred embodiment of the present invention.

5 is a graph comparing photocurrent L-I characteristics of the conventional VCSEL and the VCSEL of the present invention.

6 is a graph illustrating L-I characteristics of a conventional laser diode.

7 illustrates L-I characteristics according to an embodiment of the present invention.

8, 9, and 10 are graphs showing threshold polarization dependent on power measurements in accordance with embodiments of the present invention.

11 is a graph comparing sub-threshold polarization depending on the L-I characteristics of a conventional diode and the diode of the present invention.

12 is a graph of sub-threshold polarization ratio measurements of conventional laser diodes and inventive laser diodes.

FIG. 13 is a graph showing a cross-sectional view through line radiation showing polarization dependence of output power according to an embodiment of the present invention.

14 is a graph showing the measured polarized cavity loss measurement of the laser diode according to the embodiment of the present invention.

15 is a graph showing the sub-threaded optical spectrum of the laser diode device according to the embodiment of the present invention.

16 is a plan view of a second laser diode according to an embodiment of the present invention.

17 is a plan view of a third laser diode according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

14: upper mirror structure 15: lower mirror structure

16 active region 17 substrate

18: electrical contact

TECHNICAL FIELD The present invention relates to a laser device, and more particularly, to a method of optical polarization of a vertical cavity surface emitting laser device such as, for example, a vertical cavity surface emitting laser (VCSEL) diode.

For example, laser devices such as laser diodes are variously applied to various fields such as communication and data storage devices, and their use is increasing. One type of laser diode is a vertical cavity surface emitting laser diode (VCSEL), an embodiment of which is shown in the plan view of FIG. 1 and in the cross-sectional view of FIG.

The VCSEL 1 is composed of a substrate 7 having an electrical contact portion 8 disposed thereunder. The active region 6 is interposed between the Bragg reflector (DBR) mirror structures 4 and 5 disposed on the upper and lower parts, and the lower mirror structure is formed on the substrate 7. These mirror structures 4, 5 are configured to provide a high reflectivity of 99.5%, for example. The upper DBR mirror structure 4 includes a quantum injection region 9. This quantum injection region 9 is configured to limit the current supplied to the device.

The upper electrical contact 3 is configured on top of the upper mirror structure 4. This upper electrical contact 3 defines a hole 2 penetrating the upper surface of the upper mirror structure 4.

The VCSEL 1 of FIGS. 1 and 2 generates light output radiated in the direction of the arrow through the hole 2. The light thus output is generated by a laser effect or a laser effect in the active region 6 between two DBR mirrors 4 and 5, as is already known.

One of the main advantages of the VCSEL device is that the light to be output is generated in a direction perpendicular to the horizontal plane of the device. This is different from conventional edge emitting laser diodes that emit light into the device's horizontal plane. Therefore, the VCSEL device can be easily manufactured in various arrangements because several devices can be configured on one semiconductor region without the need to isolate the device from other devices. In particular, VCSELs are suitably used to generate circular beam light. Such circular beams eliminate the need for further optical processing prior to application to devices such as, for example, CD ROM drives or communication devices.

However, the laser radiation of the VCSEL enables orthogonal polarization or elliptic polarization by reliably controlling the direction of the polarization or polarization of individual lasing modes or filaments, which can often vary with bias current. It can also change the direction of the polarization axis even when made of the same wafer and interposed between adjacent lasers.

VCSELs are of great value in terms of their applications in a variety of applications, including optical storage, printing and communications technologies. However, in order to apply to such a field, in particular, a single transverse mode operation is required so that the polarization state is fixed in a precise direction and a high quality beam is required. The VCSELs may be formed to generate circular beams, or may be integrated into two-dimensional arrays, but are strongly affected by optical nonlinearities related to high gain operation and strong thermal effects. Such nonlinearity does not have a strong effect on the filament area across the radiation region as well as on the multi-mode operation. Eventually, this phenomenon causes the beam, which can vary depending on temperature and bias current, to be generated by peak radiation of various intensities emitted in poorly defined polarization and by fields far from poor.

In order to control the polarization state of the light emitted from the surface emission laser, there has been a conventional method using anisotropic cavity geometry that can distort the output beam shape. Methods of introducing anisotropic stresses, gains or losses have been used to control the polarization along a predefined direction, although complete control is not always maintained over the entire operating range of the device.

Temperatures that have not been tuned into non-regressive cavity mode have been used to fully control the polarization state, but in this case, control is performed at a reduced gain, which not only increases the threshold current and power consumption of the device, The result is a reduction in the useful power output. This effect is strongly dependent on the spectral disruption of orthogonal polarization, which can be affected by the residual strain of the device. Thus, to ensure that the gain difference between the two states is sufficiently achieved, another technique for controlling spectral fragmentation is required.

For example, additional configuration of other structures to generate a lattice-shaped laser can complicate the device but provide a foundation for controlling the polarization state and can affect the spatial output characteristics of laser radiation. .

Accordingly, an object of the present invention is to provide an optical polarization method capable of effectively controlling optical polarization of a vertical cavity surface emitting laser device.

According to an embodiment of the present invention for achieving the above object, a step of arranging a substrate having an electrical contact portion disposed below; Disposing a lower mirror structure on the substrate; disposing an active region on the lower mirror structure; Disposing an upper mirror structure by the active region; And disposing a contact area on the upper mirror structure to discontinuously form the inside of the body such that the polarization direction of the emitted light is substantially aligned with the boundary line of the discontinuous line.

And forming a vacuum inside the body.

Forming a trench region in the body extending from an outer surface of the device.

The vacuum region or the trench region may be filled with a material different from that of the device manufacturing material.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 and 4 are plan and cross-sectional views of the VCSEL 10 in accordance with the preferred embodiment of the present invention.

As mentioned in the conventional device description, the VCSEL 10 includes a pair of electrodes 13 and 18; Substrate material 17; And an active region 16 interposed between the upper and lower DBR mirror structures 14 and 15. The upper DBR mirror structure 15 includes a quantum implantation region 19. The upper electrode 13 defines a circular hole 12 so that the laser light radiated in the direction of the arrow passes.

The VCSELs shown in FIGS. 3 and 4 operate substantially the same as the conventional apparatus shown in FIGS. 1 and 2. In other words, the laser effect is generated in the active region 16 interposed between the two mirror structures 14 and 15. Light generated by this laser effect is emitted through the hole 12 as described above.

However, in the present invention, polarization control of the emitted laser light is achieved by introducing a discontinuous line into the device. In the arrangement shown in FIG. 4, discontinuities are provided by trenches 11 which are etched through the top electrode 13 in the DBR mirror structure 14 toward the active region 16.

This etched trench close to the lasing hole is preferably located at a distance of at least 10 microns from the edge of the hole.

Trench discontinuities can be constructed by masking and etching techniques or other techniques such as micromachining, laser ablation, e-beam lithography, X-rays or reactive ion etching (RIE), and can be constructed in a complete device.

The geometry and location needed to perform etching can also yield desirable results.

Discontinuities can be constructed by means other than trenches. For example, a vacuum may be formed inside the diode structure, or a material different from the mechanical structure, such as a metal or an oxide, may be used. The trench (or vacuum) is filled with the same material. The discontinuity lines constructed as described above are merely examples, and skilled artisans may modify and alter the discontinuity lines within the scope of the present invention.

The direction of the discontinuities can be altered to tune the polarization state for radiation in the desired / required direction. The profile angle of the discontinuous line formed from the standard position to the horizontal radial plane of the device can be varied and its depth can also be changed. It may also be formed to pass through other layers such as active region 16 and lower mirror structure 15.

The refractive index, width, length and shape of the discontinuous line 11 can also be changed.

By constructing the trenches, stress and strain can be empirically corrected by a device that controls the polarization dependence of the emitted light beams. It also results in a boundary effect that controls polarization.

The construction of the trench also triggers the additional effect that the polarization dependence is responsible for the optical mode intensity, resulting in spontaneous emission phenomena indicating similar polarization dependence on the laser radiation.

Using a trench or other discontinuity line, the superior polarization axis can be set in any desired direction, regardless of the pre-existing effects such as the crystal axis direction.

The polarization of the output light beam is aligned in the direction of the boundary line closest to the discontinuity line for the hole.

As is common knowledge, deeply formed trenches have more polarizing pinning effects than shallowly formed trenches. Trenches close to the holes 12 will have more effect than trenches spaced from the holes.

The method for achieving polarization control can be applied to laser devices that operate optically and electrically.

The semiconductor material used in the device according to the preferred embodiment of the present invention allows radiation in the wavelength range of 400 nm to 4000 nm.

The polarization control of the present invention not only significantly reduces space utilization, spectrum quality and power output characteristics of the laser, but also does not degrade the electrical characteristics of the laser as described below.

5, 6, and 7 show that the use of the polarization control method not only degrades optical power output / current characteristics, optical spectrum or spatial output, but also does not significantly degrade the critical operating performance of the device.

8 shows the L-I characteristics of a quantum implanted VCSEL in which only one polarization operation occurs.

Here, it can be seen that when the high-order transverse mode is started, the polarization state is switched, and both polarization phenomena are generated even at a high current.

9 and 10 are diagrams showing how the radiation pattern in one polarized state is tightly suppressed over the entire operating range in the apparatus according to the embodiment of the present invention, and the threshold depends on the power measurement. The influence of polarization control by polarization is shown.

After etching the trench 7 microns from the edge of the hole with a width of 1 micron and 4 to 4.5 microns deep, a single polarization operation is performed (FIG. 9).

Such an operation can be accomplished while varying the threshold current or device efficiency to a minimum range. The radial polarization state remains fixed throughout the bias current operating range of the device by the E field polarization parallel to the etching direction. It can be seen that the polarization extinction ratio is achieved above 100 (FIG. 10) and only severely decreases when thermal radiation is initiated. Etching the trench seriously polarized the spontaneous emission, triggering 1.5 times the extinction ratio under low current driving around the etch, and increasing to 3 at the etch. The pinning mechanism, unlike other devices, is very robust, allowing a single polarization operation to be performed immediately if the device remains pulsed rather than initially polarized before a steady state equilibrium is achieved for an uncontrolled device. Despite the approach of the holes, no adverse effects were found during the lifetime.

This technique has been widely applied and has always been found to be successful. In all cases, the polarization direction is aligned in the etch direction and remains fixed in a direction within the 5 degree range, which is at least the limit angle to determine the accuracy of the measurement.

11 and 12 show uneven splitting between two polarization states, including when the bias current is below the threshold during modification, sub-threading the cavity of each modified VCSEL device. The influence state of the state etched with polarization-dependent power is shown.

FIG. 13 shows that power measurements spontaneously radiated from etched lines have a high dependency on polarization.

FIG. 14 shows an example of introducing a polarization-dependent cavity loss in relation to the radiation width of the sub-threshold line, and FIG. 15 shows the spectral disruption of the orthogonal polarization state induced by changing the laser.

Since the etch state close to the radiating hole does not strongly affect the lasing mode, high quality spectral circuit output beam characteristics can be obtained for monomode devices as well as for single-mode operating regimes of multimode devices. Also, if this is done before polarized peening, longitudinal monomode operation is maintained (FIG. 15). Although small at 0.2 nm, a small shift in line width was observed, and superior polarization was observed at shorter wavelengths. This indicates that the polarization pinning mechanism can improve the brightness of the system.

Successful pinning for polarization requires that the trench be formed to a sufficient depth and must be constructed in the semiconductor material through at least the upper metallic contact layer. The minimum depth needed for pinning depends on the distance of the trench from the hole and becomes the direction of the trench relative to the direction of polarization present because it must overcome the polarization sorting mechanism in which the pinning mechanism exists before polarization is established in its own direction. The depth should also be well suited for multimode devices to demonstrate that they are polarized under any mode, and should be kept in mind that often orthogonal polarization occurs in higher order modes.

The cavity loss associated with each polarization state is estimated by measuring the polarization-resolved line width below the threshold. It can be seen from FIG. 14 that the cavity loss measured empirically by the superior polarization state is smaller than the loss in the significantly suppressed state. No pre-etch spectral splitting was observed, and the joint losses in the two polarized states were found to be similar. Optical injection measurements in which the photogenerated current from the VCSEL is measured when a detector is used have been shown to exhibit absorption depending on polarization after etching, which suggests that modifications at the band-edge are preferably made by the strain dependent effect. Indicates.

The laser diode embodied in the present invention may employ one or more discontinuities as shown in FIGS. 16 and 17. In Figure 16 two parallel discontinuities are shown, one on each side of the hole 22 of the device. 17 shows a device with a pair of orthogonal edges.

In the embodiment shown in FIG. 16, the polarization direction will be aligned in the direction of two discontinuities. The embodiment of FIG. 17 may be set such that the common gain of two orthogonal polarizations is the same.

As described above, according to the optical polarization method of the laser device of the present invention, the light emitted from the laser device can be effectively polarized by the discontinuous line to improve the spatial output characteristics of the laser radiation.

Claims (4)

Disposing a substrate 17 having an electrical contact portion 18 disposed thereunder; Disposing a lower mirror structure (15) on the substrate (17); Disposing an active region (16) in the lower mirror structure (15); Disposing an upper mirror structure (14) by the active region (16); And Disposing the contact area 13 on the upper mirror structure 14 to discontinuously form the interior of the body 13, 14, 15, 16, 17, 18, 20, 30. And polarized light in such a manner that the polarization direction of the emitted light is substantially aligned with the boundary line of the discontinuous line. The method of claim 1, comprising forming a vacuum inside the body. The method of claim 1, comprising forming a trench region in the body extending from an outer surface of the device. The method of claim 2 or 3, wherein the vacuum region or the trench region is filled with a material different from the device fabrication material.

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