Low coherence surface emitting laser (LCSEL)

Abstract

The embodiment of the invention relates to a semiconductor surface emitting light source comprising a random asymmetric cavity. Compared with a traditional vertical cavity surface emitting laser (VCSEL), the light generated by the design provided by the invention is relatively low in coherence, so that the density of coherence artifacts (speckles) in an emitted light beam is relatively low. This is desirable for imaging, picture display, holography, and tomography applications. Embodiments of the present invention provide different designs that can reduce temporal and / or spatial coherence of light by disrupting symmetry of cavities of semiconductor light sources.

Description

Technical Field

[0001] The present invention relates to semiconductor surface-emitting light sources comprising a random asymmetric cavity. Compared to conventional vertical-cavity surface-emitting lasers (VCSELs), the designs proposed herein produce light with lower coherence and, therefore, a lower density of coherence artifacts (speckle) in the emitted beam. This is desirable for imaging, picture display, holography and tomography applications. The present invention provides different designs that can reduce the temporal and / or spatial coherence of light by destroying the symmetry of the cavity of the semiconductor light source. Background Art

[0002] Vertical Cavity Surface Emitting Lasers (VCSELs) have gained a lot of attention from both academia and industry due to their advantages over traditional edge-emitting lasers. These advantages include ease of mass production by fabricating arrays of a large number of devices on the same wafer, while allowing on-wafer testing through the light output configuration. This makes them a more affordable laser choice for consumer electronics. The unique design of VCSELs also enables single-longitudinal mode emission due to the short cavity involved, which allows the free spectral range to be increased beyond the width of the active region gain distribution. By reducing the aperture ( ), single-transverse-mode VCSELs can also be designed. However, even for multimode VCSELs, the output light produced is highly coherent, which introduces speckle noise in the output beam, making it unsuitable for a wide range of applications, including display, lighting, imaging, holography, and optical coherence tomography.

[0003] For edge-emitting semiconductor light sources, the problem of high coherence is overcome by suppressing lasing by making one of the faces non-reflective, thereby preventing the formation of a Fabry-Perot resonator. Such devices that operate based on amplified spontaneous emission are called superluminescent diodes (SLDs). When population inversion is achieved in this device, amplified spontaneous emission occurs, but stimulated emission is suppressed because the photons are emitted from the face of low reflectivity. However, in a surface-emitting configuration, the same effect cannot be achieved because the active region is very thin, limiting the overall amplification of light in the active region. This poses a challenge to designing a device that can provide the advantages provided by SLDs in a surface-emitting configuration.

[0004] SLDs have found widespread use in a variety of applications due to their unique properties. Since they rely on amplified spontaneous emission, they do not suffer from the efficiency drop found in light-emitting diodes (LEDs). In addition, their modulation bandwidth is wider than that of LEDs, making them more suitable for optical communications in free space and underwater environments. SLDs have a smaller divergence angle, making them suitable for fiber coupling in fiber-optic communications. On the other hand, SLDs have lower temporal coherence than laser diodes (broader spectral emission), and their resolution in optical coherence tomography is significantly improved due to their shorter coherence length. In addition, the lower coherence results in lower speckle density in their emission, which is ideal in imaging, illumination, and picture display applications. Summary of the invention

[0005] The present invention provides a method of manufacturing a semiconductor low-coherence surface-emitting laser (LCSEL) that eliminates resonances of only a few modes while maintaining high gain in the active region. This is achieved by designing the device to pass through the active region multiple times in random directions to increase gain. Such devices can support a large number of lateral modes while maintaining a narrower emission spectrum than LEDs, thus making these devices more suitable for displays with a wide color gamut. High efficiency and low divergence provide higher brightness per pixel. The surface-emitting design greatly simplifies the manufacture of large arrays required for displays. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 A cross-sectional view of an embodiment of a low coherence surface emitting laser employing a porous layer, a dielectric top layer distributed Bragg reflector, and intracavity injection is shown.

[0007] Figure 2A cross-sectional view of an embodiment of a low coherence surface emitting laser employing a porous layer and a p-type top distributed Bragg reflector is shown.

[0008] Figure 3 A cross-sectional view of an embodiment of a low coherence surface emitting laser employing a random top reflector is shown.

[0009] Figure 4 A top view of an embodiment of a low coherence surface emitting laser employing a chaotic stadium-shaped cavity is shown.

[0010] Figure 5 A top view of an embodiment of a low coherence surface emitting laser employing a chaotic D-shaped cavity is shown.

[0011] Figure 6 A comparison between the external quantum efficiencies of conventional VCSELs and chaotic D-shaped VCSELs is shown.

[0012] Figure 7 The near-field intensity distributions of the conventional VCSEL and the chaotic D-shaped VCSEL are shown.

[0013] Figure 8 The optical spectrum of a conventional VCSEL at different injection currents is shown.

[0014] Fig. 9 The optical spectrum of the chaotic D-shaped VCSEL at different injection currents is shown.

[0015] Fig.10 The probability density functions of the intensity in the speckle pattern of the conventional VCSEL and the chaotic D-shaped VCSEL at their respective maximum power values ​​are shown.

[0016] Fig.11 A top view of an embodiment of a low coherence surface emitting laser employing a chaotic stadium-shaped cavity and a laterally coupled cavity is shown. DETAILED DESCRIPTION

[0017] Figure 1The embodiment of the present invention shown includes an n-type substrate 101, an n-type buffer layer 102, an n-type metal layer 103, an n-type distributed Bragg reflector (DBR) 104, an n-type spacer layer 105, an active region 106 consisting of multiple quantum wells and quantum barriers, a p-type oxide layer 107, a porous p-type spacer layer 108, a p-type contact layer 109, a top DBR 110, a sidewall dielectric isolation layer 111, and a p-type metal layer 112. In an embodiment, the substrate 101 and the buffer layer 102 can be made of n-type GaAs. The n-type DBR 104 can be composed of a lattice-matched n-type AlGaAs layer, in which the Al composition is alternating (e.g., alternating between 5% and 90%) to produce a periodic change in the refractive index. The n-type spacer layer 105 can be n-type GaAs, and the active region 106 can be a strained multi-quantum well InGaAs / GaAs(P) structure. The oxide layer 107 may be an AlGaAs layer with a high Al content (greater than 95%). The p-type spacer layer 108 may be a porous p-type GaAs, and the p-type contact layer 109 may be a highly doped p-type GaAs. ++ The top DBR layer 110 can be formed by depositing alternating dielectric materials such as silicon oxide and silicon nitride, where the optical thickness of each layer is equal to one quarter of the emission wavelength of the active region 106. In this case, intracavity implantation is required, where the top metal layer surrounds the top DBR layer 110. Figure 2 In another embodiment shown, alternatively, the top DBR layer 110 ′ may be composed of p-type AlGaAs with an alternating Al content. In this case, the DBR 110 ′ may be placed below the p-type contact layer 109 and the top metal layer 112 .

[0018] The sidewall dielectric isolation layer 111 can be a silicon oxide layer, which provides electrical isolation between the p-type metal layer 112 and the sidewall of the n-type layer. The function of the spacer layers 105, 108 sandwiching the active area 106 is to ensure that the active area 106 is placed at the antinode of the standing wave formed in the optical cavity. When the outer ring of the device is oxidized, the oxide layer 107 defines the aperture of the device. This is because the oxide layer defines the current injection area because the oxidized part is non-conductive. The oxide layer also provides light confinement due to the change in refractive index after oxidation.

[0019] The randomness caused by the porous p-type layer 108 in the above embodiment helps to reduce the coherence of the device. By designing the holes to be comparable in size to the wavelength of light, the holes can cause random scattering of light waves within the cavity. In addition, the random variation in refractive index caused by the presence of holes provides a random optical path length between the bottom and top DBRs 110, which changes the spectral position of the cavity resonance peak in the device. Therefore, the coherence of the emitted light will be limited.

[0020] The porous p-type spacer layer 108 can be porous p-type GaAs. In this case, the holes are formed by electrochemical anodic etching in HF:C2H5OH solution. During the etching process, the applied voltage can control the diameter of the hole, where higher voltages result in larger holes. In another embodiment, the p-type spacer 108 is made uniform (without holes). In contrast, holes are introduced into the n-type spacer layer 105, which will provide the same function.

[0021] Figure 3 Another embodiment of the invention shown may include a top surface designed to have random roughness. The scale of the roughness is close to the scale of the wavelength of light emitted by the device. The random cavities are formed by depositing a reflective metal 113 on the top surface of the rough dielectric layer 115. The roughness on the top surface can be formed by etching a large portion of the porous p-type layer 108 or by depositing a random layer of a dielectric (e.g., silicon oxide). The dielectric selected must have a wider bandgap than the bandgap of the active region to prevent fundamental absorption of light.

[0022] In another embodiment, no holes are formed in the spacer layer. Instead, incoherence is introduced by changing the lateral shape of the active region 106. This can be achieved by changing the shape of the device mesa 114 or by modifying the oxide layer 107 (current aperture). By designing the cavity laterally to support more lateral modes, the spatial coherence of the emitted light can be reduced. This is because different lateral modes are incoherent with each other. In addition, since the wavelengths in different lateral modes are slightly different, the emitted light also exhibits limited temporal coherence, even if it only supports a few longitudinal modes.

[0023] To achieve multimode emission, the modes must have similar thresholds. When all modes have similar cavity quality factors ( This can be achieved when the factor is small and the mode competition is minimized. Chaotic cavity design can be used to satisfy these conditions and maximize the number of transverse modes. An example of a chaotic cavity is Figure 4 The stadium-shaped device shown is formed by connecting two semicircles with a rectangle. Another example is Figure 5 The D-shaped cavity in the device is formed by a straight line cutting a circle at half its radius. The chaotic ray dynamics in this device prevent any single-mode The factor is significantly higher than that of other modes while limiting the gain competition between them. As multiple modes spatially cover a larger portion of the active area, the efficiency of the device is expected to be higher. In addition, due to the Purcell effect, the efficiency can also be improved due to a higher optical state density, which is defined as the number of modes divided by the spectral bandwidth spanned by the modes.

[0024] Some applications require lower spatial coherence. For example, in imaging and illumination applications, higher spatial coherence leads to an increased density of speckles (coherence artifacts), which degrades image quality. These artifacts are caused by the interference of differently coherent photons reaching the object to be imaged. Each transverse mode produces a speckle pattern. The speckle patterns of all transverse modes from a multimode transmitter increase in intensity (since they are mutually incoherent, they do not add to the field amplitude). The result is a lower speckle density.

[0025] The parameter used to quantify speckle density is speckle contrast, which is given by:

[0026] ,

[0027] in is the standard deviation of the intensity recorded by the camera, is its average value. As the number of mutually incoherent transverse modes increases, the speckle contrast decreases according to the following relationship:

[0028] ,

[0029] in is the number of speckle patterns and is equal to the number of incoherent modes. Speckle contrast values ​​below 0.03 are below human perception. To achieve this value, approximately 1,100 transverse modes are required. While this can be achieved by using a single single-mode VCSEL in an array, this approach is inefficient. In addition, the chip size required for such an array of more than 1,000 incoherent VCSELs would be large. This is because there needs to be sufficient spacing between adjacent VCSELs to avoid coupling between them. As the number of modes emitted by each individual VCSEL increases, the number of elements in the array decreases.

[0030] To show the potential of chaotic cavities in increasing optical power and the number of transverse modes, two VCSEL shapes were fabricated: a conventional circular VCSEL (O-shaped VCSEL) and a chaotic cavity D-shaped VCSEL with similar aperture area (the D-shaped VCSEL aperture is estimated to be about 7% smaller). The external quantum efficiency of these two designs at different currents is shown in Figure 2. Figure 6 As shown, Figure 6The D-shaped VCSEL shows higher values. This is attributed to the more uniform intensity distribution in the gain medium and the increased number of modes, which may increase the optical state density. The near-field intensity distribution of these two designs is shown in Figure 7 The chaotic cavity D-shaped VCSEL obviously has a more uniform distribution, so that more gain medium can contribute to the emitted optical power.

[0031] To show the increase in the number of transverse modes, the optical spectrum and spatial coherence of the VCSEL were tested. The optical spectrum of the O-type VCSEL at different currents is shown in Figure 2. Figure 8 As shown, the spectrum of D-shaped VCSEL is as follows Fig. 9 As shown, Fig. 9 A wider spectrum is shown. The full width at half maximum of the D-shaped VCSEL is about 4.5 nm, while the full width at half maximum of the O-shaped VCSEL is about 1.1 nm. In addition, the speckle pattern of the VCSEL is imaged after the light passes through the diffuser. By calculating the speckle contrast value, it is estimated that the D-shaped VCSEL emits about 97 modes. On the other hand, the conventional O-shaped VCSEL emits 51 modes. Fig.10 Shown by the average Normalized intensity value of the speckle pattern The wider distribution associated with the O-shaped VCSEL indicates that the generated speckle leads to less uniform illumination, which is caused by the higher spatial coherence of the emitted light.

[0032] Increasing the size of the device allows for a higher number of incoherent transverse modes. It also allows for higher achievable optical powers. However, the increase in size increases the junction capacitance, which will result in a slower response due to the increased RC time constant. This limits the modulation bandwidth of the device, making it unsuitable for high-speed applications such as optical wireless communications and light detection and ranging (LiDAR). Therefore, simply increasing the size of the device is not a practical approach when designing LCSELs for high-speed applications. To increase the speed of the device, a laterally coupled cavity can be used in a larger device. In this design, Fig.11 As shown in Figure 1, the device is divided into two electrically isolated parts by ion implantation. The signal is fed into the first part, while the other part is only injected with bias current.

[0033] The combination of high-quality illumination and high-speed operation of LCSELs makes them ideal for simultaneous illumination and wireless communications. Because LCSELs can be easily arranged into arrays, multiplexing and multiple-input / multiple-output schemes are possible. For example, using color conversion, wavelength division multiplexing can be achieved. In addition, using external optics, angle diversity systems can be built to provide beamforming capabilities indoors.

[0034] The ease of manufacturing large arrays of devices also provides other benefits. For example, it makes it simple and cost-effective to manufacture large quantities of red-green-blue (RGB) displays based on LCSEL designs. Since the spectral line width of LCSELs will still be much narrower than that of LEDs, this will enable higher color accuracy in RGB displays with a wider color gamut.

[0035] Another advantage of LCSELs is that they have lower temporal coherence than VCSELs and edge-emitting lasers, which is necessary for interferometry-based sensors such as optical coherence tomography (OCT) and fiber-optic gyroscopes (FOG). In OCT, for example, the achievable axial resolution is higher for sources with wider spectral linewidths. In the LCSEL design here, this is achieved due to the wavelength differences between different incoherent transverse modes.

Claims

1. A low coherence surface emitting laser, comprising: Round table top; n-type substrate; n-type buffer layer; n-type metal layer; n-type distributed Bragg reflector DBR layer; n-type spacer layer; Multiple quantum well MQW active region; Oxide layer; A porous p-type spacer layer; p-type contact layer; Dielectric DBR; Dielectric sidewall spacers; p-type metal layer.

2. The low coherence surface emitting laser according to claim 1, characterized in that: The n-type substrate, the n-type buffer layer, the n-type spacer layer and the p-type contact layer are made of GaAs, the n-type DBR layer is made of AlGaAs with an Al composition alternating between 5% and 90%, the MQW active region is made of InGaAs quantum wells and GaAsP quantum barriers, the oxide layer is made of AlGaAs with an Al composition of 98%, the pores in the p-type spacer layer are formed using electrochemical anodic etching, and the dielectric DBR is composed of alternating layers of silicon oxide and silicon nitride.

3. The low coherence surface emitting laser according to claim 1, characterized in that: The voids are generated in the n-type spacer layer but not in the p-type spacer layer.

4. The low coherence surface emitting laser according to claim 1, characterized in that: The dielectric DBR layer is replaced by a p-type AlGaAs layer with an Al composition alternating between 5% and 90% placed below the p-type contact layer.

5. A low coherence surface emitting laser comprising: Round table top; n-type substrate; n-type buffer layer; n-type metal layer; n-type DBR layer; n-type spacer layer; MQW active area; Oxide layer; p-type spacer layer; p-type contact layer; Dielectric rough top surface; Metal rough reflective layer; Dielectric sidewall spacers; p-type metal layer.

6. The low coherence surface emitting laser according to claim 5, characterized in that: The n-type substrate, the n-type buffer layer, the n-type spacer layer and the p-type contact layer are made of GaAs, the n-type DBR layer is made of AlGaAs with an Al content alternating between 5% and 90%, the MQW active region is made of InGaAs quantum wells and GaAsP quantum barriers, the oxide layer is made of AlGaAs with an Al content of 98%, and the dielectric rough surface is made of silicon oxide.

7. A low coherence surface emitting laser comprising: Chaotic cavity table; n-type substrate; n-type buffer layer; n-type metal layer; n-type DBR layer; n-type spacer layer; MQW active area; Oxide layer; A porous p-type spacer layer; p-type contact layer; p-type DBR; Dielectric sidewall spacers; p-type metal layer.

8. The low coherence surface emitting laser according to claim 7, characterized in that: The n-type substrate, the n-type buffer layer, the n-type spacer layer and the p-type contact layer are made of GaAs, the n-type DBR layer and the p-type DBR layer are made of AlGaAs with an Al composition alternating between 5% and 90%, the MQW active region is made of InGaAs quantum wells and GaAsP quantum barriers, and the oxide layer is made of AlGaAs with an Al composition of 98%.

9. The low coherence surface emitting laser according to claim 7, characterized in that: The chaotic cavity is in the shape of a stadium.

10. The low coherence surface emitting laser according to claim 7, characterized in that: The chaotic cavity is D-shaped.

11. The low coherence surface emitting laser according to claim 8, characterized in that: The mesa is divided into two electrically isolated sections, forming a laterally coupled cavity for high-speed operation.

12. The low coherence surface emitting laser according to claim 11, characterized in that: The electrical isolation is achieved by ion implantation.

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