Distributed feedback semiconductor laser including wavelength monitoring section

In general, a complex-coupled distributed feedback (DFB) semiconductor laser includes a grating formed by grooves through at least a part of an active region of a laser cavity. The complex-coupled DFB laser may be configured with a wavelength monitoring section and may be configured to provide facet power asymmetry. The wavelength monitoring section may include a second-order grating section configured to emit radiation (e.g., vertical radiation) from a side of the DFB laser for monitoring.

TECHNICAL FIELD

The present invention relates to distributed feedback (DFB) lasers and in particular, to DFB semiconductor lasers including a wavelength monitoring section.

BACKGROUND INFORMATION

Semiconductor lasers may be used in a variety of industrial and scientific applications, such as optical communications. Optical communications applications, for example, may employ lasers that emit light at a particular lasing wavelength (e.g., 1.31 μm or 1.55 μm) suitable for transmission through optical fibers. Semiconductor lasers may be desirable over other types of lasers because they have a relatively small volume and consume a relatively small amount of power.

Lasers generally include a laser cavity defined by mirrors or reflectors and an optical gain medium between the reflectors in the laser cavity. When pumped with pumping energy (e.g., an electrical current), the gain medium amplifies electromagnetic waves (e.g., light) in the cavity by stimulated emission, thereby providing optical gain and generating a laser light output. In semiconductor lasers, a semiconductor active layer or region serves as the gain medium and reflectors provide optical feedback for laser oscillation within the active region. In Fabry-Perot lasers, for example, a set of mirrors or cleaved facets bound the active region to provide the optical feedback. In other semiconductor lasers, such as distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers, one or more diffraction gratings (e.g., Bragg gratings) may be used to provide reflectance. In a DFB laser, for example, a distributed reflector (e.g., a diffraction grating or Bragg grating) along the active region provides the optical feedback and may be used to restrict oscillation to a single mode.

Fiber optic communication systems may require a high performance light source capable of generating single-mode, narrow spectral linewidth emission in the 1.3-1.56 μm wavelength range. Some of the existing semiconductor lasers (e.g., InGaAsP DFB lasers) fail to provide stable single-mode operation that is insensitive to ambient temperature change (uncooled operation) and insensitive to external optical feedback (isolator-less operation) and fail to provide high single-mode yield and high output power. Complex-coupled DFB lasers have been developed that provide advantages such as high single-mode yield, less sensitivity to external optical feedback, high modulation bandwidth and reduced wavelength chirp. Complex-coupled DFB lasers generally provide both index coupling and gain coupling. Certain complex-coupled DFB lasers will predominantly lase on the longer wavelength side of the Bragg stop band (this lasing mode is referred to as the long Bragg mode hereafter). In complex-coupled DFB lasers with periodically etched MQW's, however, there are still fundamental problems such as variations of the complex coupling coefficient due to variations of grating etching depth, laser performance variations due to random variations of facet grating phase, and variations in lasing wavelength due to the ratio of index to gain coupling.

Semiconductor lasers may be designed to provide wavelength monitoring. In some semiconductor lasers, for example, wavelength monitoring may be performed out of a back facet of the laser. Such wavelength monitoring out of the back facet, however, requires cleaving of the laser. Thus, wavelength monitoring may not be performed during wafer testing before the lasers are cleaved.

DETAILED DESCRIPTION

In general, a distributed feedback (DFB) semiconductor laser includes a grating formed by grooves through at least a part of an active region of a laser cavity. The DFB laser may be configured with a wavelength monitoring section and may be configured to provide facet power asymmetry. As will be described in greater detail below, the wavelength monitoring section may include a second-order grating section configured to emit radiation (e.g., vertical radiation) from a side of the DFB laser for monitoring. In an embodiment, the DFB laser may also have a complex coupling coefficient.

Referring toFIG. 1, a DFB laser device100with a wavelength monitoring section, consistent with an embodiment, is described in greater detail. The DFB laser device100includes an active region110with periodic variation of thickness to form a grating120. The active region110may include quantum-well (QW) layers111,113,115,117,119and barrier layers112,114,116,118. The grating120is formed by grooves122extending at least partially through the active region110, for example, through some of the QW layers117,119and into one of the barrier layers116. The grooves122may be spaced along at least a portion of the active region110(only a section of the DFB laser device100is shown inFIG. 1).

The laser device100may also include a semiconductor substrate102(e.g., an N-type InP substrate) on which is grown a buffer layer104(e.g., 1.5 μm thick layer of N-type InP). A first confinement region130may be provided over the buffer layer104. The first confinement region may include three confinement layers132,134, and136, for example, of N-type InGaAsP with band gap energies corresponding to wavelengths of 1.0 μm, 1.05 μm, and 1.1 μm, respectively. The thickness of each of the confinement layers132,134,136may be about 20 nm.

The active region110overlies the confinement region130and may include five QW layers111,113,115,117,119and four barrier layers112,114,116,118. The barrier layers112,114,118may be made of InGaAsP composition with a band gap energy corresponding to a wavelength of 1.1 μm and may each be about 10 nm thick. The QW layers111,113,119may be made of InGaAsP composition and may be tailored so as to provide a first transition energy of the QW, corresponding to a wavelength of 1.31 μm. The barrier layer116may be made of InP with the layer being 10˜20 nm thick. The QW layers115and117may be made of InGaAsP composition and may be tailored so as to provide a first transition energy of the QW, corresponding to a wavelength of 1.31 μm. The QW transition energy may be determined by a combination of the QW alloy composition (e.g., having a PL peak wavelength of around 1.3 μm), strain (e.g., compressive strain of 1%), thickness (e.g., 6˜7 nm), and the band gap energy of their adjacent barrier layers (e.g., a band gap energy corresponding to a wavelength of 1.1 μm).

A second confinement region150may be provided on top of the active region110. The second confinement region150may include two confinement layers152,154of InGaAsP with band gap energies corresponding to wavelengths of 1.1 μm, and 1.0 μm, respectively, and each with a layer thickness of about 20 nm.

The grating120may be defined by periodically etching the grooves122, for example, through the QW layers117,119and stopping in the barrier layer116. The grating120may be a Bragg grating in which a grating period or pitch A of the grating is related to the Bragg wavelength λBas follows:
λB,q=2neΛ/q
where neis the effective index of refraction and q is an integer (q=1, 2, . . . ). The pitch Λ of the grooves of the grating120may be selected so as to define an order of the grating, as will be described in greater detail below. A fill layer160having substantially the same band gap energy as the barrier layer116may fill the grooves122of the grating120. The fill layer160may be a P-type InP material grown to fill the grooves and to make a layer of 0.2 μm thickness on top of the confinement layer154.

An upper cladding layer162may be provided on the fill layer160followed by a highly doped P-type capping layer164for contact enhancement. The upper cladding layer162may be a layer of P-type InP having a thickness of 1.3˜1.6 μm and the capping layer164may be a layer of InGaAs having a thickness 200 nm. The laser device100may also include top electrode170and bottom electrode172for current injection into the laser device100.

In operation, the wavelength corresponding to a transition between the first quantized levels of conduction and valance bands of a QW with symmetric barriers is given by the following equation:

λ⁡(µm)=1.24Eg+Ec+Ev(1)
where Egis the band gap energy of the QW (in unit eV), and Ecand Evare the quantized energy levels in the conduction and valence bands, respectively (in unit eV). Eccan be approximated as

Ec=[π2⁢acW+Δ⁢⁢Wc]2⁢⁢where(2)Δ⁢⁢Wc=acbc⁢Δ⁢⁢Ec(3)ac=2⁢ℏ2⁢mcw(4)bc=mcwmcb(5)
ΔEcis the discontinuity of the band edge of the conduction band, h is Planck's constant divided by 2π, W is the QW width, and mcwand mcbare the effective masses of electrons inside the well, and the barrier, respectively. The quantized energy Evfor the valance band can be expressed by equation (2) replacing subscript “c” with a subscript “v”. It can be seen from equation (2) that the larger ΔEc(ΔEv) gives larger Ec(Ev), which in turn gives shorter first transition wavelength λ in equation (1). Thus, the barrier having larger band gap energy has a shorter first transition wavelength for a given QW.

According to the exemplary embodiment, the QW layers111,113,119may include, for example, a lattice matched quaternary (Q) InGaAsP QW layer of band gap energy of 0.886 eV and thickness 65 nm and with a corresponding wavelength of 1.4 μm (also referred to as a 1.4 Q). The QW layers111,113,119may have symmetric 1.1 Q barrier layers112,114,118, and, in this example, the first transition wavelength of this symmetric QW structure is calculated to be 1.313 μm. The other two QW layers115,117may have asymmetric barrier layers, for example, 1.1 Q barrier layers114,118and barrier layer116of InP with a band gap energy of 1.35 eV. The first transition wavelength of this asymmetric QW structure is calculated numerically to be 1.300 μm. For comparison, the first transition wavelength of a symmetric QW structure with symmetric InP is calculated by the equations above to be 1.279 μm. Therefore, the first transition wavelength (1.300 μm) of the asymmetric QW structure with 1.1 Q and InP barriers is between those of the symmetric QW structures with 1.1 Q and InP barriers (i.e., between the wavelengths 1.313 μm and 1.279 μm, respectively). Thus, the first transition wavelength of a QW may be made shorter by using a barrier layer of larger band gap energy. The symmetric and asymmetric quantum-well structures may provide the same gain peak wavelength in order to have a sufficient modal gain for lasing. The first transition wavelength of the two asymmetric QW structures can be increased from 1.300 μm to 1.313 μm by increasing the QW thickness slightly. In this way, the two types of quantum-well structures may be tailored to provide almost same gain peak wavelengths.

A DFB laser device with a wavelength monitoring section, consistent with embodiments described herein, may also have other configurations and layers and may have other grating structures. In particular, other numbers and types of QW layers and barrier layers may be used. Also, although a triangular or trapezoidal shape is shown for the grating, other shapes may also be used such as square or sinusoidal.

FIG. 2shows a complex-coupled DFB laser device200, consistent with an embodiment, configured to provide wavelength monitoring and facet power asymmetry. The complex-coupled DFB laser device200includes a laser cavity202extending between front and back facets204,206and first and second sides207,209. The laser cavity202may include first and second main cavity sections202-1,202-2with first and second complex-coupled grating sections220-1,220-2, respectively, having different Bragg wavelengths (i.e., λgb, λgf). In the exemplary embodiment, the laser cavity sections202-1,202-2are located in the back and front of the laser device200, respectively, such that the back laser cavity section202-1includes the first grating section220-1and the front laser cavity section202-2includes the second grating section220-2.

The laser cavity202further includes a third or wavelength monitoring cavity section202-3with a second order grating section220-3, which may have a Bragg wavelength (λgf) of the second grating section220-2in the second or front cavity section202-2. The second order grating section220-3in the wavelength monitoring cavity section202-3is configured to emit radiation (e.g., vertical radiation) from the first side207of the DFB laser device200. The vertical radiation may be at the same wavelength as the lasing wavelength and used for monitoring the wavelength of the DFB laser device200.

In this embodiment, the first and second grating sections220-1,220-2are first-order gratings and generally have a grating period or pitch of one-half wavelength (i.e., λgb/2 and λgf/2 respectively), whereas the second-order grating section220-3has a grating period or pitch of one wavelength (λgf). In other embodiments, the laser200may include additional laser cavity sections with additional grating sections. The grating sections220-1,220-2,220-3may be formed with different grating periods, for example, by electron beam writing and changing the grating pitch along the cavity length.

In the exemplary embodiment, the lengths of the laser cavity sections202-1,202-2may be selected such that the second grating section220-2(e.g., in the second or front main cavity section202-2) is longer and provides the main feedback mechanism for lasing. The first grating section220-1(e.g., in the first or back main cavity section202-1) is shorter and acts as a reflector for the lasing wavelength. The Bragg wavelength λgbof the first or back grating section220-1may be greater than the Bragg wavelength λgfof the second or front grating section220-2. The Bragg wavelength λgbmay be, for example, about 1˜2 nm greater than the Bragg wavelength λgf. Because of complex coupling in this embodiment, the lasing mode is obtained at the longer wavelength side of the Bragg stop band of the front grating section220-2. The front and back facets204,206may be AR (antireflection) coated to eliminate facet reflections.

The grating sections220-1,220-2,220-3may be formed in an active region210of the laser cavity202, for example, as described above and shown inFIG. 1. The grating sections220-1,220-2,220-3thus each include alternating low corrugation regions222-1,222-2,222-3and high corrugation regions224-1,224-2,224-3. The duty cycle of the grating sections may be equal to the ratio of the length of the low corrugation regions to the length of the high corrugation regions. In the exemplary embodiment shown inFIG. 2, the duty cycle of the first and second grating sections220-1,220-2is about 50%. The length of the high and low corrugation sections222-1,224-1in the first grating section is about λgb/4 and the length of the high and low corrugations sections222-2,224-2in the second grating section220-2is about λgf/4, where λgb/4>λgf/4. As used herein, the term “about” allows a variation within acceptable tolerances.

The duty cycle of the second-order grating section220-3is about 25%, which provides a higher vertical radiation component. In the second-order grating section220-3, the length of the high corrugation sections222-3is about λgf/4 and the length of the low corrugation sections224-3is about 3λgf/4. The second-order grating section220-3in the wavelength monitoring section202-3may also have other duty cycles capable of providing sufficient radiation from the first side207for monitoring the wavelength.

The low corrugation regions222-1,222-2,222-3and the high corrugation regions224-1,224-2,224-3provide alternating regions with complex indices, NLand NH, respectively, which are a function of the QWs in the corrugation region. The complex indices, NLand NH, may be expressed as follows:
Ns=ns+jms(6)
where nsand msare the real part and imaginary part of the complex index for the section s (s=H or L) and mscan be expressed as
ms=[Γsgs−(1−Γs)αs]/(2k)  (7)
where Γsis the optical confinement factor for the active QW, gsis the material gain in the QW, αsis the absorption loss in other layer, and k is the vacuum wavenumber. In the exemplary embodiment, the refractive index nHof the high corrugation regions224-1,224-2,224-3is larger than the refractive index nLof the low corrugation regions222-1,222-2,222-3because the high corrugation regions have more quaternary materials which have a higher refractive index. In the exemplary embodiment, the optical confinement factor ΓHof the high corrugation regions224-1,224-2,224-3is larger than the optical confinement factor ΓLof the low corrugation regions222-1,222-2,222-3because the high corrugation region has a larger number of QWs and Γsis roughly proportional to the number of QWs. The absorption loss αsis also usually smaller than the material gain gsresulting in mH>mL.

In general, the difference in the real parts nHand nLmay provide index coupling and the difference in the imaginary parts mHand mLmay provide gain coupling. There are two dominant modes at both edges of the Bragg stop band (the long Bragg mode and the short Bragg mode) for index-coupled DFB lasers with no facet reflections. The field of the long Bragg mode is mainly confined in the high corrugation region, while the field of the short Bragg mode is mainly confined in the low corrugation region. If the high index region has a higher gain (mH>mLin equation (7)) (in-phase complex coupling), the long Bragg mode experiences a higher gain than the short Bragg mode. Therefore, the long Bragg mode becomes the main mode, and the short Bragg mode is suppressed. On the contrary, if the high index region has a lower gain (anti-phase complex coupling), the short Bragg mode becomes the main mode. In the exemplary embodiment ofFIG. 2, the high corrugation region has higher nHand nL. Therefore, the structure has in-phase complex coupling, which provides high single-mode stability.

Since the main mode is determined mainly by the front cavity section202-2(which is chosen to be 70˜80% of the total length of the main cavity sections), the mode wavelength is located at the longer wavelength side of the Bragg stop band of second grating section220-2in the front main cavity section202-2. In this embodiment, the lasing mode is obtained at the longer wavelength side of the Bragg stop band of the second grating section220-1due to in-phase complex coupling. The lasing wavelength is given approximately by the λgf+Δλgf/2 where Δλgfis the width of the Bragg stop band of the second grating section220-2in the front main cavity section202-2. Therefore, if λgbis chosen such that λgb≈λgf+Δλgf/2, the lasing wavelength is made to match λgbat which the first grating section220-1in the back main cavity section202-1has a maximum reflection. In this way, an asymmetric power distribution in the laser cavity is produced, making the front facet power higher than the back facet power, which reduces or eliminates the random effect of facet grating phase.

FIG. 3shows a gain coupled DFB laser300that provides wavelength monitoring, consistent with another embodiment. The gain coupled DFB laser300includes a laser cavity302extending between front and back facets304,306and first and second sides307,309. The laser cavity302may include first and second main cavity sections302-1,302-2with first-order and second-order grating sections320-1,320-2, respectively. The gain coupled DFB laser300also includes a third or wavelength cavity section302-3with a second-order grating section320-3that emits radiation from the first side307for wavelength monitoring, as described above. The grating sections320-1,320-2,320-3may be formed in an active region310of the laser cavity302, for example, as described above and shown inFIG. 1.

In the exemplary embodiment, the main cavity sections302-1,302-2are located in the back and front of the laser device300, respectively, such that the back main cavity section302-1includes the first-order grating section320-1and the front main cavity section302-2includes the second-order grating section320-2. The first-order grating section320-1generally has a grating period or pitch of one-half wavelength (λgf/2), whereas the second-order grating section320-2has a grating period or pitch of one wavelength (λgf). In other embodiments, the laser300may include additional laser cavity sections with additional grating sections. One example of a gain coupled DFB laser with a second-order grating used as the main feedback mechanism is described in greater detail in U.S. patent application Ser. No. 12/119,586, which is commonly owned and filed concurrently herewith.

In the exemplary embodiment shown inFIG. 3, the second-order grating section320-2has a duty cycle of substantially 50%, which causes the index coupling coefficient to become substantially zero and substantially eliminates the vertical radiation component in the second-order grating section320-2. Thus, the index coupling provided by the quantum well etching in the second-order grating section320-2does not contribute to feedback in the grating direction and only gain coupling is provided by the modulation of quantum well numbers. The resulting substantially pure gain coupling gives a single mode oscillation at the Bragg wavelength λgf of the second-order grating section320-2, which may improve wavelength accuracy.

Referring toFIG. 4, a DFB laser400including a monitoring section, according to any of the embodiments described above, may be used in a laser transmitter402including a laser drive circuit410. The complex coupled DFB laser400may be electrically coupled to the laser drive circuit410and optically coupled to an optical waveguide (not shown) such as an optical fiber. One example of the laser transmitter402is a laser transmitter designed for optical fiber communication applications, such as the type available from Applied Optoelectronics, Inc. for use in CATV communications. In such “broadband” applications, the laser transmitter402and particularly the DFB laser400may be designed for high bit rate operation, for example, up to about 10 Gb/s and the DFB laser400may be configured for operation at wavelengths such as 1310 nm and 1550 nm. This example of the laser transmitter402may also include other components, such as an RF amplifier, a thermoelectric cooler (TEC) controller, a microcontroller, a predistortion circuit, and/or a clipping correction circuit, as well as other components known to those skilled in the art for use in a laser transmitter. Embodiments of the DFB laser with wavelength monitoring may also be used in other types of laser transmitters used in other communications applications or in other non-communications applications, such as chemical sensing.

The laser drive circuit410may include circuitry known to those skilled in the art for providing at least a modulation current412to the DFB laser400. The laser drive circuit410may also provide other currents to the DFB laser400such as a laser threshold current and/or a bias current. The DFB laser400receives the modulation current412and generates a modulated light output420in response to the modulation current412. Thus, the modulation of the light occurs within the cavity of the DFB laser400in this embodiment. When providing direct modulation in the laser transmitter302, a more stable single-mode operation and improved single-mode yield and output power may be achieved by using the DFB laser400and the wavelength may be more easily monitored using the wavelength monitoring section.

Accordingly, embodiments of the DFB laser with wavelength monitoring facilitate wavelength monitoring while also improving single-mode stability, reducing the effects of facet grating phase, and/or improving wavelength accuracy. Consistent with one embodiment, a distributed feedback (DFB) semiconductor laser device includes a laser cavity extending from a front facet to a rear facet and between first and second sides. The laser cavity includes at least first and second main cavity sections and a wavelength monitoring section. The DFB laser may also include an active region located in the laser cavity. The active region includes a plurality of quantum-well layers and barrier layers between the quantum-well layers. The bandgap energy of the barrier layers is greater than the bandgap energy of the quantum-well layers. The DFB laser further includes a grating formed by grooves extending at least partially through the active region and spaced along at least a portion of the laser cavity. The grating includes first and second grating sections in the first and second main cavity sections, respectively, and a second-order grating section in the wavelength monitoring section of the cavity. The second grating section is longer than the first grating section and provides a main feedback mechanism for lasing. The first grating section is configured to reflect the lasing wavelength such that laser radiation is emitted from the front facet. The second-order grating section in the wavelength monitoring section is configured to emit laser radiation from the first side.

Consistent with another embodiment, a laser transmitter includes a laser drive circuit configured to provide at least a modulation current and a directly modulated distributed feedback semiconductor laser configured to receive the modulation current and configured to generate a modulated light output in response to the modulation current. The directly modulated distributed feedback semiconductor laser includes a laser cavity extending from a front facet to a rear facet and between first and second sides. The laser cavity includes at least first and second main cavity sections and a wavelength monitoring section. The DFB laser may also include an active region located in the laser cavity. The active region includes a plurality of quantum-well layers and barrier layers between the quantum-well layers. The bandgap energy of the barrier layers is greater than the bandgap energy of the quantum-well layers. The DFB laser further includes a grating formed by grooves extending at least partially through the active region and spaced along at least a portion of the laser cavity. The grating includes first and second grating sections in the first and second main cavity sections, respectively, and a second-order grating section in the wavelength monitoring section of the cavity. The second grating section is longer than the first grating section and provides a main feedback mechanism for lasing. The first grating section is configured to reflect the lasing wavelength such that laser radiation is emitted from the front facet. The second-order grating section in the wavelength monitoring section is configured to emit laser radiation from the first side.