External cavity tunable optical transmitters

External cavity optical transmitters are disclosed which include a gain chip and a mirror that define an optical cavity. The optical cavity may include collimating lenses, electro-optic crystals for controlled phase shift, and additional frequency selective optical components such as a grating and/or an etalon. The external cavity laser is capable of lasing at any of the specified ITU wavelengths (frequencies) in the L, C, or any other band in the optical spectrum. The optical transmitter is wavelength tuned by a lens mounted on an actuator.

TECHNICAL FIELD

The present disclosure pertains to optical systems and, more particularly, to external cavity tunable optical transmitters.

BACKGROUND

Optical systems are widely used in communications applications to facilitate the exchange of information such as voice and data over fiber cable, which may be fabricated from glass or any other suitable composite material. Both telephony and Internet-based systems exploit the wide bandwidth and large data capacity that optical systems provide. Additionally, as compared to conventional wired systems, optical networks are easily maintained and repaired.

Conventional optical systems include a transmitter having a laser that operates at or near one of the wavelengths specified by the International Telecommunications Union (ITU). The laser could be an external cavity laser having an optical cavity, a grating, and an etalon. In such an arrangement, the grating coarsely tunes the laser and the etalon finely tunes the laser. As will be readily appreciated by those having ordinary skill in the art, the optical length of the cavity in which a laser operates and the free spectral ranges of the grating and the etalon affect the wavelength at which the laser lases. Accordingly, as the dimensions of the laser cavity change, the operating wavelength of the laser drifts, resulting in reduced power output from the laser and potentially in mode hopping of the laser. Additionally, it is possible to fabricate lasers capable of operation at a number of different wavelengths that are spaced evenly with respect to one another. For example, etalons have free spectral ranges of 25, 50, and 100 Gigahertz (GHz), which allow lasers to be designed to operate within these frequency spacings.

To address wavelength drift and to allow for wavelength tuning of lasers, gratings of previous external cavity lasers were pivotable about their axes. The pivotable nature of the gratings allowed the grating to steer a particular wavelength of interest so that it would be reflected from the grating at an angle that would cause the optical energy to reflect into the optical cavity for lasing.

DETAILED DESCRIPTION

Although the following discloses example systems including, among other components, software executed on hardware, it should be noted that such systems are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of these components could be implemented using dedicated hardware, exclusively in software, exclusively in firmware or in some combination of hardware, firmware and/or software. Accordingly, while the following describes example systems, persons of ordinary skill in the art will readily appreciate that the examples are not the only way to implement such systems.

Turning now toFIG. 1, an external cavity optical transmitter (optical transmitter)100includes a substrate102on which a number of components are disposed to form an external cavity laser. In particular, a gain chip104having first and second reflective coatings106,108, a first lens110, a second lens112, an actuator113on which the first lens110is mounted, a grating114and an etalon116, which may be combined with a mirror118to form a resonant optical cavity. In one example, the second reflective coating108is an anti-reflection coating having a reflectivity less than 1 E-5. The second lens112is disposed between the first reflective coating106of gain chip104and a coupling122. The coupling122may be an optical fiber, an optical modulator, or any other suitable optical component. The grating114, etalon116, mirror118, and the first lens110are mounted in a manner that leaves them relatively thermally insensitive so that the temperature of the gain chip104may be varied without affecting the other components. In an alternate implementation, the gain chip104may be operated at a fixed temperature when using thermally sensitive components in the transmitter100.

The optical transmitter100further includes an electro-optic crystal (EO)124disposed between the first lens110and the grating114. In operation, as a bias voltage on the EO124is changed, the refractive index though the EO124changes, thereby changing the effective optical path length between the gain chip104and the mirror118, which, in turn, enables single mode hop-free operation of the external cavity laser at the chosen wavelength.

As shown inFIG. 1, there are various electrical components associated with the optical transmitter100. The electrical devices may include a back facet detector125, a thermistor126, and a thermal-electric cooler (TEC) controller128, which is coupled to a TEC129. The optical transmitter100further includes a processing unit130and an associated memory131. The processing unit130is coupled to the actuator113, the back facet detector125, the thermistor126, the TEC controller128, and is further coupled to a voltage source132. The voltage source132is coupled to the actuator113and the EO124. Additionally or alternatively, the transmitter100may include a front facet detector134, which receives optical energy from a beam splitter136, and is coupled to the processing unit130. In such an arrangement, the beam splitter136could be implemented using, for example, a 5% beam splitter.

The actuator113may be implemented using a voice coil-type actuator having a physical displacement that varies as a function of the voltage that is applied thereto. In one example, the actuator113may be displaceable in two dimensions and may be a dual axis voice coil. For example, the actuator113may have a relative position of zero when no voltage is applied thereto, but may have a relative position of one millimeter (mm) when one volt is applied thereto. Alternatively, the range of movement could be anywhere between 50 microns and 1000 microns over a one volt or sub-one volt input range. As an alternative to voice coil technology, the actuator113could be implemented using actuators of other types or technologies, such as, for example, piezo-electric actuators.

It should be noted that while the processing unit130and the memory131are shown as separate components inFIG. 1, those having ordinary skill in the art will readily recognize that such a representation is merely one example configuration of a processing unit and its associated memory. For example, the processing unit130and the memory131could be integrated into one single processing unit including on-board memory.

Generally, during operation, when power is applied to the gain chip104, the gain chip104emits optical energy through the second reflective coating108and the first lens110transfers the emitted optical energy to the grating114. The grating114separates the optical energy into its constituent wavelengths and reflects a wavelength of interest to the mirror118through the etalon116. This is represented inFIG. 1by the ray lines140. The optical energy emitted from the gain chip104at the wavelength of interest is reflected from the grating at an angle θ, which is perpendicular to the surface of the mirror118. The energy reflected at angle θ reaches the mirror118and is reflected back from the mirror118to the grating114and through the first lens110to the gain chip104. Accordingly, the gain chip104and the mirror118form an optical resonant cavity in which lasing at the wavelength of interest occurs. In such an arrangement, the grating114performs coarse filtering and the etalon performs fine filtering at the ITU frequencies. Optical energy that is not at the wavelength of interest reflects from the grating114to the mirror118at an angle such that the mirror118will not reflect that optical energy back to the gain chip104, but instead reflects the undesired optical energy in a different direction. The reflection of wavelengths that are not of interest is represented inFIG. 1by the ray lines142.

While the gain chip104, the grating114and the mirror118form the optical resonant cavity, the gain chip104, the second lens112and the coupling122form the output of the optical transmitter10. In particular, during operation, optical energy from the gain chip104, which is a result of the lasing between the first reflective coating106and the mirror118, is emitted from the gain chip104through the first reflective coating106and coupled to the coupling122via the second lens112. As shown inFIG. 1, the optical transmitter100may include a modulation source145. The source145may be separate from, or a part of, the substrate102, and provides an information signal to the coupling122that may cause the coupling122to modulate the intensity of the optical energy.

The processing unit130is also able to control the voltage source132to change the voltage bias applied to the EO124to alter the refractive index thereof. Accordingly, the effective optical path length, between the gain chip104and the mirror118changes with the bias voltage applied to the EO124. Additionally, the processing unit130controls the actuator113to change the position of the first lens110to select a particular wavelength for lasing. Further details pertinent to the operational aspects of the optical transmitter100are now provided in conjunction withFIGS. 2-5.

As will be readily appreciated by those having ordinary skill in the art, various ones of the optical components (e.g., the gain chip104, the grating114or the mirror116) can change positions over time and over temperature. Positional fluctuation of the optical components alters the length of the optical cavity and the optical alignment of the system, thereby changing the lasing wavelength, output power, and modal stability of the system100. As disclosed herein, because the first lens110is mounted on the actuator113, the position of the first lens110may be varied to tune the lasing wavelength of the system100. Additionally, the ability to change the physical location of the first lens110enables the processing unit130, via the actuator113, to select a particular wavelength for lasing.

For example, as shown inFIGS. 2-4, varying positions of the first lens110along x or y-axes (where the z-axis is the optical axis and the x and y-axes are mutually orthogonal thereto) results in a desired wavelength being reflected at angle θ and being normally incident on the mirror.FIG. 2represents the position of the first lens110before the optical components have shifted positions. The rays of optical energy, two of which are shown at reference numeral202, emitted by the gain chip104and are collimated by the first lens110into parallel rays of optical energy, two of which are shown at reference numeral204. As each ray of collimated optical energy204strikes the grating114, each ray is split into constituent wavelengths of λo, λo−δ and λo+δ. In one particular example, λo, λo−δ could be wavelengths corresponding to frequency spacings of 25, 50 and 100 GHz, respectively. BecauseFIG. 2represents a situation in which no optical components have shifted, the wavelength of λois reflected from the grating at the angle θ and this wavelength is the only wavelength normally incident on the mirror118.

As shown inFIG. 3, movement of the first lens110downward, due to the actuator113, causes rays of optical energy306from the gain chip104to be collimated by the first lens110into rays308that impact the grating114at a less acute angle (i.e., at an angle that is more normal to the grating114) than the rays204ofFIG. 2. Due to Snell's Law, the wavelength of λoalways reflects from the grating114at an angle that is equal to its angle of incidence. Accordingly, the shift of the lens110causes the wavelength λo+δ to be reflected at the angle θ.

Conversely, as shown inFIG. 4, movement of the first lens110upward, due to the actuator113, causes rays of optical energy410from the gain chip to be collimated by the first lens110into rays162that impact the grating114at a more acute angle (i.e., at an angle that is less normal to the grating114) than the rays204ofFIG. 2. Again, due to Snell's Law, λois reflected at an angle equal to its angle of incidence, thereby causing the wavelength of λo−δ to be reflected at the angle θ.

As shown inFIGS. 2-4, it is possible to move the first lens110via the actuator113to change the wavelength that is reflected at the angle θ, which is normal to the mirror116. For example, if the alignment in the optical cavity between the gain chip104and the mirror116were to change and thereby cause the wavelength of λo+δ to be reflected at the angle θ, it is possible to move the first lens110upward to change the angle of incidence and to cause the wavelength of λoto be reflected from the grating at the angle θ. Additionally, it is possible to move the first lens110to select one of a number of wavelengths for lasing. For example, if the etalon116(FIG. 1) were designed to have free spectral ranges of 25, 50, and 100 GHz, the first lens110could be moved to select any one of the free spectral ranges for lasing by steering the desired free spectral range to reflect from the grating114at the angle θ. Accordingly, the movement of the first lens110position, which may be linear movement, allows the system100to be wavelength tuned by beam-steering optical energy from the gain chip104, without the need to change the rotational position of the grating114.

The EO crystal124, which changes the phase of the optical beam of energy passing through as the voltage applied thereto changes, is used to maintain optimum path length control of the external cavity laser. As the gain chip ages, the current required to maintain optical output power increases and hence causes phase variations to the optical beam emanating from the gain chip. The EO crystal124can control the phase and hence compensate for aging of the gain chip. In addition, the actuator and mirror can react to external forces, the EO crystal phase shift compensates for the movement of the lens and maintain the external cavity path length constant. The actuator113and EO crystal124are designed such that the EO crystal124can fully compensate for the movement of the actuator113in response to external force(s).

As shown inFIG. 5, a plot500of operating current502against EO voltage includes a point on the plot representing the minimal operating current is designated with reference numeral504. At the point504, the operating current is at a minimum and, therefore, the gain chip104is operating at its optimal efficiency. In this example, the minimum operating current is determined by taking one or more derivatives of the operating current502with respect to EO voltage. The minimum point504is a point having a second derivative value of zero that is bounded by plot portions having negative and positive first derivatives. The range denoted by reference numeral506is the range over which the search for the minimum point504is carried out and the range denoted by reference numeral508is a range that may be considered to be a lock threshold that is sufficiently close to the minimum point504.

Turning now toFIG. 6, a tune routine600, which maybe stored in memory131and executed by the processing unit130, is represented in block diagram format. Upon power-up, the processing unit130sets the substrate102, via the TEC129, to a stored temperature value (block602). After the TEC temperature is set, the operating current associated with the gain chip104is set to a stored value (block604).

After the operating current is set (block604), the processing unit130and the voltage source132cooperate to apply a voltage ramp to the actuator113(block606). The voltage ramp varies the position of the first lens110, thereby steering the optical energy from the gain chip104as it passes to the grating114. The variation in the first lens110position changes the wavelength and the optical energy that is coupled to the back facet detector135and front facet detector134.

As the voltage ramp is applied to the actuator113, the processing unit130measures via the front facet detector134or the back facet detector135the output power of the gain chip104as a function of the position of the first lens110(block610). As the processing unit130monitors the output power of the gain chip104, the processing unit130counts the number of output power peaks that are detected (block610). For example, as shown inFIG. 7, a plot700shows power output versus first lens110position. Reference numerals702and704represent peaks in the output power that may be due to the resonant properties of the optical cavity. For example, the first peak702may be located at a lens position corresponding to a reflected wavelength at an ITU specified wavelength. The second peak704may be located at a lens position corresponding to a reflected wavelength with a frequency separation of 100 GHz with respect to the first peak702. Power peaks at these lens positions may be due to the etalon116having free spectral ranges of 50 or 100 GHz, respectively, and the difference in the magnitudes between the first and second peaks702,704is due to the path length not being optimized for all the selected wavelengths.

The processing unit130is informed, a priori, of the etalon116characteristics and the EO crystal voltage ramp to keep cavity length approximately constant as the actuator113moves. Accordingly, the processing unit130is able to determine, based on an analysis of the peaks in the power output plot700, the wavelengths that correspond to the two lens positions that yield the peaks702,704. Accordingly, starting with a first lens position of zero and ramping the voltage on the actuator113to move the first lens110to its farthest position, the processing unit130determines the lens positions at which peaks occur, merely by counting the peaks experienced by the front facet detector134or the back facet detector135as the voltage to the actuator113is ramped.

After the processing unit130determines power output as a function of the actuator voltage (block606), the processing unit130sets the actuator113to the position yielding the desired lasing wavelength, which is one of the peaks in the power output plot702(block612). When the actuator113is set to the desired position, the first lens110is in a position that beam-steers the desired wavelength to impact the grating114at an angle that causes the desired wavelength to be reflected from the grating at the desired angle. The processing unit130measures the power output as determined by the front facet detector134or the back facet detector135(block616) and monitors the power output to keep the power output at a desired level (block618). The processing unit130maintains operation at block618as long as the power output is at the desired level. If the power output drops below or rises above the desired level, the processing unit will again apply the voltage ramp to the actuator113(block606).

Turning now toFIG. 8, a process by which the optical transmitter100may be fabricated is shown at reference numeral800. The process begins by placing a lens (e.g., the first lens110) on an actuator (e.g., the actuator113) and placing both the actuator and its associated lens on a substrate (e.g., the substrate102) (block802). The gain chip (e.g., the gain chip104) is then placed on the substrate102. As the gain chip is placed on the substrate using a pick and place machine, the x-axis position of the gain chip is controlled tightly. The z-axis position of the lens110is then selected using the actuator113to move the lens110(block806). The divergence of the beam is then measured (block808). The z-axis position of the lens110is then optimized using the actuator113to minimize the divergence and to collimate the light beam from the gain chip104(block810).

The EO124and the etalon116are then placed on the substrate102between the lens110, which, as discussed above, is disposed on the actuator113, and the mirror118, which has tilt control about its y-axis (blocks812and814). The tilt of the mirror118is adjusted to start the external cavity lasing at any ITU channel and to maximize the output power of the laser (block816). The threshold current of the gain chip104is then minimized using the actuator113to vary the y and z-axis positions of the lens110(block818). The path length control loop, which varies the voltage bias on the EO124to adjust path length, is then enabled (block820).

Referring back to block810, divergence is minimized when operating current is minimized. As shown inFIG. 9, a plot900of operating current902against z-axis lens position includes a point on the plot representing the minimal operating current is designated with reference numeral904. At the point904, the operating current is at a minimum and, therefore, divergence is a minimum. In this example, the minimum operating current is determined by taking one or more derivatives of the operating current902with respect to the z-axis position. The minimum point904is a point having a second derivative value of zero that is bounded by plot portions having negative and positive first derivatives. The range denoted by reference numeral906is the range over which the search for the minimum point904is carried out and the range denoted by reference numeral908is a range that may be considered to be a lock threshold that is sufficiently close to the minimum point904.

If the laser wavelength is accurate to within, for example, 1 GHz of the ITU wavelength specification (block822), the laser is then wavelength tuned by translating the lens110in, for example, the y-axis, using the actuator113(block824). Alternatively, if the laser wavelength is not within 5 picometers of the ITU wavelength specification, the tilt of the mirror118is changed (block816), the threshold current is minimized by changing the lens position (block818) and the path length control loop is enabled (block820).

The laser wavelength is then tested to determine if it is accurate to within 5 picometers at the beginning, middle, and end of, for example, the C and/or L bands (block826). Alternatively, any other band in the optical range may be selected for use. If the laser wavelength is not accurate to within 5 picometers, the tilt of the mirror118is adjusted (block816) and the threshold current is minimized by varying the lens position (block818) and the path length control loop is enabled (block820). When the laser wavelength is accurate to within 5 picometers at the beginning, middle and end of the C/L band (block326), the mirror118is fixed in place (block828) and the threshold current is minimized by changing the position of the lens110in the y and z-axes via the actuator113to compensate for post-fix movement of the mirror118(block830).

After the lens is fixed in place (block828) and the threshold current is minimized (block830), the second lens (e.g., the lens112) is aligned to couple light from the gain chip104to the coupling122, which includes the beam splitter136and the front facet detector134. The second lens is then welded in place on the substrate102(block832). After the lens112or system of lenses has been fixed in place, the substrate102is placed on the TEC129(block834) and the hermetic sealing of the package including the components ofFIG. 1is completed (block836).

After packaging is complete (block836), the tunable laser is calibrated (block838). Calibration includes, but is not limited to, determining control circuit set points for wavelength control, which is controlled by the position of the lens110on the actuator113, and path length control. Calibration may also include populating a look up table of lens positions and EO bias voltages as functions of lasing wavelengths.