Patent Description:
A vertical-cavity surface-emitting laser (VCSEL) is a semiconductor-based laser diode that can emit a highly efficient optical beam vertically, for example, from its top surface. In VCSELs, high reflectivity mirrors are generally required. The high reflectivity mirrors can be implemented, for example, as distributed Bragg reflectors (DBR) (e.g., quarter-wave-thick layers of alternating high and low refractive indexes), made of semiconductor or dielectric material. To achieve a high reflectivity with a reasonable number of layers, a high index contrast is provided (e.g., a high-contrast DBR). However, use of a high-contrast DBR can generate a broad stop-band and, in the case of VCSELs with a long internal monolithic cavity, this will allow multiple longitudinal modes to lase. The longitudinal modes can, in some applications, give rise to undesirable or unstable operation (e.g., "kinks" in the power versus current curve; mode-hoping).

Patent Application <CIT> discloses a narrow beam divergence semiconductor source comprising in particular an extended two-sections DBR.

The present disclosure describes narrow beam divergence semiconductor sources. The presence of an extended length mirror (also referred to sometimes as a hybrid mirror) can help suppress one or more longitudinal and/or transverse modes such that the beam divergence and/or the spectral width of emission is substantially reduced.

For example, in one aspect, the disclosure describes a narrow beam divergence semiconductor source operable to generate a beam having a substantially narrow beam divergence, an emission wavelength, and a substantially uniform beam intensity. The narrow beam divergence semiconductor source includes an optical resonant cavity including a high reflection mirror having first and second sides, an extended length mirror having first and second sides, and an active region. The high reflection mirror and the extended length mirror are disposed on distal sides of the active region such that the first side of the high reflection mirror is coupled to a first side of the active region and the first side of the extended length mirror is coupled to a second side of the active region opposing the first. Electrical contacts are operable to direct electric current to the active region. The extended length mirror comprises a narrow bandwidth mirror and a high-reflectivity mirror.

Some implementations include one or more of the following features. For example, the extended length mirror and the high reflection mirror can be operable to suppress one or more longitudinal and/or transverse modes. In some implementations, only one longitudinal mode lases.

The disclosed techniques can be applied to various types of narrow beam divergence semiconductor sources including, for example, VCSELs, VECSELs, LEDs and RC-LEDs.

The disclosure also describes edge-emitting light sources. For example, according to one aspect, the disclosure describes a narrow beam divergence semiconductor optical edge-emitting laser that includes a hybrid distributed Bragg reflector (hybrid DBR). The hybrid DBR has first and second sides, the edge-emitting laser being disposed on the first side of the hybrid DBR. The hybrid DBR includes a high-contrast region and a low-contrast region. The high-contrast region includes multiple high refractive index difference pairs of a DBR materials of a particular charge-carrier type, the high-contrast pairs being periodically disposed within the high-contrast region. The low-contrast region includes multiple pairs of a low refractive index difference DBR materials of the particular charge-carrier type, the low-contrast pairs being periodically disposed within the low-contrast region. The hybrid DBR and the edge-emitting laser are operable to generate an emission having a spectral width of emission and a beam divergence. The edge-emitting laser and the hybrid DBR have a narrow spectral bandwidth and are operable to substantially suppress one or more transverse and/or longitudinal modes such that the beam divergence and/or the spectral width of emission is substantially reduced.

The invention is defined by independent claim <NUM>, additional optional features being further defined by the dependent claims.

Other aspects, features and various advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claims.

The present disclosure describes VCSELs having low divergence and/or operable for high single-mode power in some cases. In particular, a hybrid mirror is provided by combining a narrow bandwidth mirror with a high-reflectivity mirror, such that the narrow bandwidth mirror is placed within the laser cavity (i.e., between two high-reflectivity mirrors). Preferably, the narrow bandwidth mirror has a sufficiently large penetration depth to achieve the desired diffraction losses of higher order transverse modes, and has a narrow enough stop-band to filter out unwanted modes. The reflectivity of the high-reflectivity mirror should be insufficient by itself for the laser to achieve lasing. There should be an adequate phase matching layer between the two mirrors for constructive interference. The combined reflectivity at the designed wavelength (peak reflectivity) is sufficient for the laser to achieve lasing.

As shown in <FIG>, a top-emitting VCSEL device <NUM> includes a substrate <NUM> (e.g., a N-GaAs substrate) on which epitaxial layers for the VCSEL structure are grown, for example, by a metal-organic chemical vapor deposition (MOCVD) or other deposition process. The optical resonant laser cavity of the VCSEL is formed by a hybrid mirror <NUM> and a distributed Bragg grating (DBR) partial-reflectivity top mirror <NUM> to allow for emission of the VCSEL beam <NUM>. The hybrid mirror <NUM> can be achieved by combining a narrow bandwidth mirror <NUM> (e.g., a low-contrast N-DBR) with a high-reflectivity (e.g., <NUM>%) bottom mirror <NUM>, such that the narrow bandwidth mirror <NUM> is placed within the laser cavity (i.e., between the two relatively high-reflectivity mirrors <NUM>, <NUM>). The bottom mirror <NUM> can be implemented, for example, as a high-contrast N-DBR. One or more phase-matching layers <NUM> can be provided between the bottom mirror <NUM> and the narrow bandwidth mirror <NUM>. The top mirror <NUM> can be implemented, for example, as a high-contrast P-DBR.

A gain section <NUM>, which may be referred to as an active section and can include quantum wells, is disposed between the hybrid reflector <NUM> and the top reflector <NUM>. A current aperture <NUM> confines the current in the center region of the VCSEL device <NUM> to activate the quantum wells to produce optical gain and to generate a laser cavity mode in the VCSEL laser cavity. In the top-emitting VCSEL device illustrated in <FIG>, the output beam <NUM> is taken out of the partial-reflectivity top mirror <NUM>.

The VCSEL device <NUM> is activated by applying current through an anode and cathode electrical connections <NUM>, <NUM>, which can be implemented, for example, as metal contacts. The presence of the low-contrast DBR in the hybrid mirror <NUM> increases the effective length of the optical resonant cavity such that multiple longitudinal modes are present. Thus, the hybrid mirror <NUM> also may be referred to as an extended length mirror. Because of the effective narrower bandwidth of the hybrid mirror <NUM>, the additional, unwanted longitudinal modes have much higher round-trip losses compared to the main mode and, thus, the longitudinal modes do not achieve lasing. Thus, the hybrid mirror <NUM> and the high reflection mirror <NUM> are operable to provide mode filtering by suppressing one or more longitudinal and/or transverse modes. Preferably, in some implementations, only one longitudinal mode lases.

Various details of the hybrid mirror <NUM> can vary depending on the implementation. Nevertheless, in a particular example, the hybrid mirror <NUM> can be composed of the following layers: a low-contrast N-DBR layer <NUM> having a thickness in a range of <NUM> - <NUM>, and a refractive index difference Δn/n in the range of <NUM>% - <NUM>%; a N-phase matching layer <NUM> having a quarter wavelength optical thickness, and an index of refraction n of about <NUM>; and a high-contrast N-DBR mirror <NUM> having a thickness in a range of <NUM> - <NUM>, and refractive index difference Δn/n in the range of <NUM>% - <NUM>%. Some or all of the foregoing values may differ for other implementations.

In some instances, the extended length mirror has an effective penetration depth extending multiple emission wavelength distances from the first side of the extended length mirror. For example, the effective penetration depth of the extended length mirror extends, in some cases, between <NUM> - <NUM> emission wavelength distances. In some cases, the penetration depth of the extended length mirror is between <NUM> - <NUM>, the emission wavelength is between <NUM> - <NUM>, and the relative refractive index difference is between <NUM> - <NUM>%. In some instances, the penetration depth of the high reflection mirror is between <NUM> - <NUM>, the emission wavelength is between <NUM> - <NUM>, and the relative refractive index difference is between <NUM> -<NUM>%.

In some instances, the high reflection mirror has an effective penetration depth extending multiple emission wavelength distances from the first side of the high reflection mirror. In some cases, the effective penetration depth of the high reflection mirror extends between <NUM> - <NUM> emission wavelength distances.

In some implementations, the full-width half-maximum (FWHM) intensity divergence angle is less than <NUM> degrees.

Some implementations include additional features to enhance operation. For example, as shown in <FIG>, the VCSEL device includes a high-contrast dielectric mirror coating <NUM> on top of a phase matching layer122 and a low-contrast mirror <NUM>.

A hybrid mirror as described above also can be integrated into a bottom-emitting VCSEL <NUM> as shown in the example of <FIG>. The VCSEL device <NUM> includes a substrate <NUM> (e.g., a N-GaAs substrate) on which epitaxial layers for the VCSEL structure are grown. The optical resonant laser cavity of the VCSEL is formed by a hybrid mirror <NUM> and a distributed Bragg grating (DBR) high-reflectivity top mirror <NUM> (e.g., <NUM>%). The hybrid mirror <NUM> can be achieved by combining a narrow bandwidth mirror <NUM> (e.g., a low-contrast N-DBR) with the partial-reflectivity bottom mirror <NUM>, such that the narrow bandwidth mirror <NUM> is placed within the laser cavity (i.e., between the two relatively high-reflectivity mirrors <NUM>, <NUM>). The bottom mirror <NUM> in this case is partially reflecting so as to allow for emission of the VCSEL beam <NUM>. The bottom mirror <NUM> can be implemented, for example, as a high-contrast N-DBR. One or more phase-matching layers <NUM> can be provided between the bottom mirror <NUM> and the narrow bandwidth mirror <NUM>. The top mirror <NUM> can be implemented, for example, as a high-contrast P-DBR.

The gain section <NUM>, which can include quantum wells, is disposed between the hybrid mirror <NUM> and the top mirror <NUM>. A current aperture <NUM> confines the current in the center region of the VCSEL device <NUM> to activate the quantum wells to produce optical gain and to generate a laser cavity mode in the VCSEL laser cavity. The VCSEL device <NUM> is activated by applying current through an anode and cathode electrical connections <NUM>, <NUM>, which can be implemented, for example, as metal contacts. In the bottom-emitting VCSEL device illustrated in <FIG>, the output beam <NUM> is taken out of the partial-reflectivity bottom mirror <NUM>.

As with the top-emitting VCSEL, the bottom-emitting VCSEL <NUM> is operable to provide mode filtering by suppressing one or more longitudinal and/or transverse modes. Preferably, in some implementations, only one longitudinal mode lases.

A low-contrast mirror can be used with other device such as vertical external-cavity surface-emitting lasers (VECSELs) as well, light emitting diodes (LEDs) and RC-LEDs. <FIG> illustrate examples.

As shown in the example of <FIG>, a low-contrast mirror <NUM> is provided on the external mirror <NUM> of a VECSEL. The low-contrast mirror <NUM> can be implemented, for example, using a shallow contrast dielectric coating.

Similarly, <FIG> shows an example of a LED <NUM> that includes a low-contrast mirror <NUM>, and <FIG> shows an example of a RC-LED <NUM> that includes a low-contrast mirror <NUM>.

Although the foregoing examples illustrate incorporation of a low-contrast mirror <NUM> or <NUM> onto vertically emitting devices, the techniques also can be used in connection with edge-emitting devices (e.g., edge-emitting lasers). As illustrated in <FIG>, a narrow beam divergence semiconductor optical edge-emitting laser <NUM> includes a hybrid mirror (e.g., a hybrid DBR) <NUM>. The hybrid DBR has first and second sides, the edge-emitting laser <NUM> being disposed on the first side of the hybrid DBR <NUM>. The hybrid DBR <NUM> includes a high-contrast region <NUM> and a low-contrast region <NUM>. The high-contrast region <NUM> includes multiple high refractive index difference pairs of DBR materials of a second charge-carrier type, the high-contrast pairs being periodically disposed within the high-contrast region. The low-contrast region <NUM> includes multiple pairs of low refractive index difference DBR materials of the second charge-carrier type, the low-contrast pairs being periodically disposed within the low-contrast region. The hybrid DBR <NUM> can include one or more phase-matching layers <NUM> disposed between the high-contrast region <NUM> and the low-contrast region <NUM>. The hybrid DBR also can include a backside dielectric coating disposed on the second side of the hybrid DBR. The hybrid DBR <NUM> and the edge-emitting laser <NUM> are operable in combination to generate a spectral bandwidth of emission <NUM>, where one or more transverse and/or longitudinal modes are substantially suppressed such that the beam divergence and/or the spectral width of emission is substantially reduced.

Claim 1:
A narrow beam divergence semiconductor source (<NUM>) operable to generate a beam having a substantially narrow beam divergence, an emission wavelength, and a substantially uniform beam intensity, the narrow beam divergence semiconductor source comprising:
an optical resonant cavity including a high reflection mirror (<NUM>) having first and second sides, an extended length mirror (<NUM>) having first and second sides, and an active region;
the high reflection mirror and the extended length mirror being disposed on distal sides of the active region such that the first side of the high reflection mirror is coupled to a first side of the active region and the first side of the extended length mirror is coupled to a second side of the active region opposing the first; and
a plurality of electrical contacts (<NUM>,<NUM>) operable to direct electric current to the active region,
characterized in that
the extended length mirror comprises a narrow bandwidth mirror (<NUM>) and a high-reflectivity mirror (<NUM>), the narrow bandwidth mirror (<NUM>) having a narrower bandwidth than the high-reflectivity mirror (<NUM>) and the high-reflectivity mirror (<NUM>) having a higher reflectivity than the narrow bandwidth mirror (<NUM>).