Patent Grant
8867579
1. Field of the Invention
The present invention relates to widely tunable semiconductor lasers of the DBR (Distributed Bragg Reflector) type, comprising at least one reflecting Bragg sections, at least one separate gain section and at least one phase section. Specifically, it relates to such a laser having reduced linewidth and frequency noise. The invention also relates to a method for reducing linewidth and frequency noise in a laser using DBR type reflectors.
2. Description of the Related Art
Conventionally, widely tunable lasers are used in Wavelength Division Multiplexed (WDM) optical transmission systems. As opposed to fixed wavelength lasers, tunable lasers can be used for several wavelengths, leading to for instance cost-efficient transmission equipment and simpler inventory management for system manufacturers. They also enable more advanced reconfigurable networks based on wavelength switching.
One group of such widely tunable lasers is the group of DBR based tunable lasers, such as DBR, MGY-DBR, SG-DBR, SSG-DBR, DS-DBR type lasers. Typically, they are made as monolithic single chips, without moving parts. This way, they can be made small and rugged. Their tunability across a broad frequency spectrum is due to a complex interplay between their Bragg section(s) and a phase section. In general, Bragg current(s) need(s) to be selected in order to define the lasing wavelength. Also, a phase current needs to be selected in order for the optical length of the laser cavity to match this lasing wavelength, so that high-power, stable lasing can be achieved. To tune the laser, the Bragg and phase currents are then adjusted simultaneously to alter the lasing wavelength. Different Bragg currents, or combinations of Bragg currents, yield locally tunable lasing within frequency intervals which are quite disparate in relation to each other. In different such tuning intervals, the properties of the laser can be different in terms of for instance the output frequency response to a small change in one of the control currents. A gain current must also be applied. DBR lasers and their tuning is described for instance in SE 1051045 A1.
Herein, the term “DBR laser” is to be understood so as to comprise all lasers using at least one DBR type reflector. Hence, the above described types of DBR lasers are non-limiting examples of such lasers. Furthermore, herein a “Bragg” section is used to denote a DBR section.
A main drawback of the DBR based laser is its relatively large frequency noise and linewidth. Especially in coherent communication system, where the information is both amplitude- and frequency modulated on the carrier light wave, this may constitute a problem. It has proven difficult to efficiently reduce the linewidth of DBR lasers, due to their complex structure.
The present invention provides a way to reliably and efficiently reduce the linewidth of DBR lasers across a wide range of lasing frequencies.
For DFB (Distributed FeedBack) lasers, negative electrical feedback to the gain laser material has been proposed to decrease the laser linewidth, for instance in U.S. Pat. No. 7,471,710 and U.S. Pat. No. 7,620,081. DFB lasers have the advantage of having a relative low linewidth, typically in the order of 10 times narrower than for a DBR laser, but are not tunable across as large a frequency range as DBR lasers. However, the findings described in these prior art documents are not applicable to DBR lasers because of a number of differences between DFB and DBR lasers.
Namely, DFB lasers typically consist of a single amplification-providing section, to which the negative feedback signal is applied. The carrier density in such a section is clamped, resulting in a certain self regulation according to which the linewidth decreases with increased output power. A DBR laser includes at least two passive sections which lack this type of self regulation, and the carrier density is therefore free to fluctuate.
Due to the gain clamping mechanism, the frequency modulation response in the gain section is dominated by the red-shifting temperature dependence of the refractive index up to frequencies in the range of around 1 MHz. Beyond this frequency, the frequency modulation response is dominated by the change of the refractive index vs. carrier density. These two effects are 180° out of phase, such that a feedback loop configured for negative feedback at frequencies below 1 MHz will turn into positive feedback at frequencies above 1 MHz.
Thus, the invention relates to a laser device comprising a laser in turn comprising at least one Distributed Bragg Reflector (DBR) section, at least one phase section and at least one gain section, wherein the laser device further comprises a laser control means, arranged to feed respective tuning currents to the said at least one DBR section, to the said at least one phase section and to at least one gain section in order to tune the laser, wherein the laser device further comprises a feed-back control means, wherein the laser device further comprises a frequency noise detector in communication with said feedback control means, arranged to detect a frequency fluctuation in a light beam output from the laser and to communicate a signal representing the frequency fluctuation to the feedback control means, which feedback control means is arranged to feed a respective variable electric feedback signal to at least one of said at least one DBR section and said at least one phase section of the laser, so that the output laser frequency is altered in response to a variation in the feedback signal or the combination of respective feedback signals, whereby the feedback signal or combination of respective feedback signals is varied as a function of the detected frequency fluctuation so as to counteract the detected frequency fluctuation.
It also relates to a method for reducing linewidth and frequency noise in a laser device comprising a laser in turn comprising at least one Distributed Bragg Reflector (DBR) section, at least one phase section and at least one gain section, which laser device further comprises a laser control means, arranged to feed respective tuning currents to the said at least one Bragg section, to the said at least one phase section and to at least one gain section in order to tune the laser.
In the following, the invention will be described in detail, with reference to the enclosed drawings, wherein:
a and
a and 1b illustrate schematically two exemplary embodiments of a Distributed Bragg Reflector (DBR) laser device 100 according to the invention, comprising a DBR laser 110, a laser control means 150 and a feedback control means 140. The laser 110 in turn comprises at least one Bragg section 111, at least one phase section 112 and at least one gain section 113.
101 represents an integrated semiconductor optical circuit, preferably in the form of a single optical sub-assembly.
Throughout the description, the invention is described as if the laser control means 150 and the feedback control means 140 are separate entities. This may or may not be the case, and as the skilled person will realize, they may also be represented by different functionality in one and the same control circuitry in practical applications.
The laser control means 150 is arranged to feed, using tuning control current sources 151, 152, 153, respective tuning currents to the said at least one Bragg section 111, to at least one of said at least one phase section 112 and to said at least one gain section 113, in order to tune the laser to a certain WDM channel, at a specific frequency and a specific output power. The different currents are supplied to the laser 110 sections 111-113 via respective electrodes 115. The control of the tuning currents supplied to the different sections 111-113 is conventional as such, and is described for instance in SE 1051045 A1.
A frequency noise discriminator 130, 131 is arranged to communicate with the feedback control means 140 and to detect a frequency fluctuation in a light beam 116 output from the DBR laser 110. The said discriminator 130, 131 is also arranged to communicate a signal representing the frequency fluctuation to the feedback control means 140.
According to a preferred embodiment, a light splitter 120 is arranged to feed part of the light beam 116 to the discriminator 130, 131, which preferably comprises a frequency discriminator means 130 and a light intensity detector means 131.
The frequency discriminator means 130 may for instance be a Fabry-Perot etalon, used for locking the frequency to a WDM channel (that is, a wavelength locker), or a Mach-Zehnder or a Michelson interferometer. It is arranged to read light output from the laser 110 and to convert a light frequency change into an intensity change in an output light beam which is fed to the light intensity detector means 131.
The means 131, in turn, may be a conventional photo detector or an avalanche photo-diode, and is arranged to read the light beam output from the frequency discriminator means 130 and to convert a detected light intensity change in the said read light beam into a change in a output electrical signal, which is then communicated to the feedback control means 140 as a signal representing the said frequency fluctuation.
According to the invention, the feedback control means 140 is then arranged to feed a respective variable electric feedback signal to at least one of said at least one Bragg section 111 and said at least one phase 112 section of the DBR laser 110, preferably either exactly one Bragg section 111 or exactly one phase section 112, preferably only to one phase section 112. The variable feedback signal or combination of variable feedback signals are selected so that the output laser frequency is altered in response to a variation in the feedback signal or the combination of feedback signals. Moreover, the feedback signal or combination of feedback signals is varied as a function of the detected frequency fluctuation so that the changes in laser 110 output frequency, due to said feedback signal or signals, counteract the frequency fluctuation detected by the frequency noise discriminator 130, 131.
Hence, respective feedback signals may be applied to either one or several Bragg sections, to one or several phase sections, or to a combination of one or several Bragg and one or several phase sections. In the following, Bragg and phase sections will collectively be denoted “passive” sections, since they, as opposed to gain sections 113, are not active in the sense that they do not contribute to any gain in the laser light.
The feedback control means 140 thus comprises analog and/or digital electronics (see below) arranged to apply an electrical feedback signal to at least one passive section of the laser 110 with the aim of suppressing the frequency noise in the output light beam 116. In order to suppress this noise, it is preferred that the feedback signal to some extent oscillates in reverse phase in relation to detected frequency fluctuations, in other words it is a negative feedback signal. However, since the frequency modulation response of a DBR laser is typically non-linear, it is important that the electric feedback signal fed to the passive section 112 also provides the right amount of magnification and the appropriate phase characteristics to efficiently suppress the said noise without causing oscillations in the feedback system.
The variable electric feedback signal can be a feedback current fed to the passive section in question, altering the frequency of the laser. Alternatively, the variable electric feedback signal can be a variable feedback reverse bias voltage applied to the passive section in question, with a similar result. In the latter case, for a reverse biased passive section the electro-optic effect is used instead of current injection to achieve frequency modulation. This will put higher amplification demands on the individual components in a device according to the invention, but on the other hand the modulation response can reach several GHz, depending on the high frequency design of the chip and its peripherals.
Which magnification and phase characteristics to use in the feedback signal is highly dependent on at least the characteristics of the DBR laser 110 and the characteristics of the feedback system 130 itself, 131, 140. Therefore, these properties of the feedback control means will have to be determined experimentally. However, what is important is that the present inventors have surprisingly discovered that it is possible to use negative feedback via one or several passive sections of a DBR laser to suppress frequency fluctuations, notably that it is not necessary to provide a negative feedback signal to all tuned sections of the laser 110, including the gain section 113.
In fact, using only passive sections, a larger negative feedback bandwidth can be achieved. Namely, the frequency modulation response has a blue shifting characteristic from DC up to its roll off frequency, which for current modulation normally is at several tens of MHz and for high biasing currents would even reach hundreds of MHz and for voltage modulation I in the GHz regime.
As compared to the DFB laser case, injecting a corrective current into the gain material affects both the intensity and the frequency of the DFB laser, requiring higher complexity in the laser circuitry. By feeding back only to passive sections, the laser light intensity is not, to first order, affected, whereby the laser control circuitry can be made less complex.
Even though Bragg and phase sections have different functions in a DBR laser, they both react to a small change in injected current by altering the lasing frequency according to predetermined respective functions. It is preferred that the frequency noise of the laser beam 116 which is corrected by the present invention has an amplitude which is less, preferably significantly less, than the largest frequency shift which may typically be imparted to the laser beam 116 light, by altering one or several of the tuning currents fed to passive sections, without risking a mode hop of the laser 110 when the laser 110 starts from a stable lasing mode.
a illustrates a method in which the feedback current is fed from the feedback control means 140, via summing means 132a, to the phase section 112. In
Since the laser 110 is widely tunable, attention must be paid to the fact that various components of the feedback loop have non-linear characteristics. For instance, the locking point of laser 110 will, for different lasing channels, occur at different points along the transmission function slope of a Fabry-Perot interferometer used as the discriminator 130. Since the frequency demodulation sensitivity of the etalon varies across a wide spectrum of frequencies, it will be different for different channels.
Similarly, the amplitude of the frequency modulation response of the laser 110 depends strongly on the DC-current level (or bias current) provided to a passive section, as can be seen in
Furthermore, the responsivity of the detector 131 is typically also weakly wavelength dependent.
Hence, the gain and phase characteristics of a feedback loop used to achieve a decreased linewidth of a DBR laser will depend upon the lasing frequency currently used, at least if the feedback signal is applied to a passive section which is also used for tuning the DBR laser.
The photodetector 131 is connected to a front end amplifier 142 with transfer function HA, and in conjunction with this detection noise δiN is introduced. This detection noise is caused by for instance the thermal Jonsson Noise, shot-noise and noise coming from the front end amplifier 142.
The signal after the front end amplifier 142 needs to be preconditioned with the appropriate phase and magnification before being fed back to the passive section 111 or 112 of the laser 110, using logic variable phase correction Φ and variable gain correction G elements, respectively. Finally, the passive laser 110 section 111 or 112 itself has the frequency modulation transfer function Hν(ω(ν)).
Thus, there are three elements in the feedback loop transfer functions of which depend on the optical frequency, in other words on the selected WDM channel for the laser 110, namely the Fabry-Perot interferometer 130, the laser 110 itself, and to some extent the responsivity of the photo detector 131, especially when using an avalanche photo diode with RPD>1.
Thus, the open loop feedback transfer function is:
wherein Pin is the optical power input into the FP etalon.
It can be shown that, under electrical feedback, the frequency noise power spectral density (PSD) SνFB(f) is:
where
is the PSD in the free running case.
In order to achieve reduction of the noise PSD and the linewidth of the laser 110, high feedback gain is required. However, care must be taken with the stability of the feedback system.
According to a preferred embodiment, as shown in
It is furthermore preferred that the feedback control means 140 is arranged to receive a signal 149 representing the presently used WDM channel of the laser 110 and/or the current lasing frequency, and that the feedback control means 140 is arranged to vary the said feedback current or combination of feedback currents so that each respective feedback current amplitude, and preferably also the phase characteristics of each respective feedback current, as described above, depends on the presently used WDM channel and/or frequency. With respect to said phase characteristics, it is in particular preferred that the phase delay between the frequency fluctuations detected in the light beam 116 and each respective feedback current fed into the passive section 111, 112 is determined as a function of the currently used lasing WDM channel. It is also preferred that the amplitude and/or phase characteristics of the feedback signal is determined based upon a known relationship between currently used lasing frequency and the transfer functions of other components 130, 131, 142 in the feedback loop apart from the laser 110 itself.
The variable phase Φ and gain G logic units in the feedback circuit can be implemented as circuits in analog electronics, or using digital signal processing.
Finally, the calculated feedback signal is again transformed into an output analog current signal 903 by a Digital to analog converter (DAC) 930, thereby continuously producing the feedback current or combination of feedback currents to be applied to the passive section 111 or 112 via 132a or 132b.
The front end amplifier 1002 can be implemented as a trans-impedance amplifier (TIA) or as an integrating high impedance amplifier. A TIA typically has a flat transfer function up to the frequency where a double pole in the complex plane occurs (See for example data sheet for Texas Instruments OPA657, http://www.ti.com/lit/ds/symlink/opa657.pdf). Beyond this frequency, the transfer function falls by 40 dB/decade, and the phase suffers a rotation of 180°. If the phase reaches 180° while the loop gain is larger than unity, the control system will become unstable. To avoid instability, the amplitude and phase conditioning circuit 1003 should be designed to correct for this double pole, for example by introducing a zero. This will reduce the amplitude slope to 20 dB/decade but will also limit the phase rotation to 90°, which will restore stability to the control loop.
An integrating front end amplifier has a relatively low frequency pole determined by the parasitic capacitance of the photo diode and the high input resistance value. Beyond this frequency, the front end behaves like an integrator with a slope of 20 dB/decade and a phase rotation of 90°, which renders stable characteristics, as explained above. On the other hand, a zero can be introduced with the amplitude and phase conditioning circuit 1003 in order to extend the flat response beyond the pole of the integrating amplifier 1002.
Other purposes of the amplitude and phase conditioning circuit circuit 1003 comprise to compensate for the roll off encountered in the transfer function of the laser tuning section, and to adjust the amplification of the control loop. These two functions can be implemented using variable gain amplifier designs and/or using networks implemented with potentiometers such that the RC-constants of the circuit can be tuned, preferably in a digital fashion. These two functions are important since different laser biases will render different laser bandwidth and tuning efficiency.
An analog input signal, representing the detected frequency fluctuation, enters on a terminal 1101, and the corrected feedback signal exits on terminal 1102 in the form of an analog current to be fed to the passive section 111, 112.
According to a preferred embodiment, it is the properties of the amplitude and phase conditioning step which are varied depending on the currently used lasing frequency or WDM channel of the laser 110.
Operation amplifier 1108 is connected as a non-inverting amplifier, and has a first order low pass filter characteristic. Operation amplifier 1116 is connected as a derivative network. A typical DBR laser has a transfer function which is similar to a first order low pass filter, with a pole the frequency of which depends on the DC current bias used. Since different selected frequencies or WDM channels have different DC current settings, the frequency of the pole will depend on the used WDM channel. With this in mind, the derivative network is designed with a zero point which is selected so as to compensate for the said pole in the laser 110 transfer function.
The purpose of the emitter following transistor step 1120 is to achieve an amplification of the signal which is independent of the laser impedance.
In order to adapt the gain G and phase Φ of the output signal, for instance resistors 1114 and 1118 can be made variable and set to suitable values depending on the desired gain G and phase Φ. Alternatively, resistor 1118 and capacitor 1115 can be made variable and varied with the same purpose.
1112 and 1113 are current sources. The following table shows an exemplifying set of resistor and capacitor dimensions for use in a particular exemplary case, but the skilled person realizes that the values given need to be adapted to each specific case in order to fully achieve the advantages of the present invention:
Thus,
Since the phase section 213 is not used for tuning the laser 210 to a specific WDM channel, the frequency response to a changed feedback current is independent of the currently used lasing frequency or WDM channel. Therefore, G and Φ need not be changed to reflect changes in a transfer function of the laser 210 itself in response to a change of WDM channel, which simplifies the control circuit 241.
In
This way, WDM channel independency of the transfer function of the laser 310 is achieved with respect to the feedback current, in a way which is similar to the one described in connection to
One possible problem of devices 100, 200 and 300 is that the photo detector 131, 231, 331 will also be susceptible to intensity fluctuations in the output power of the laser 110, 210, 310. If these fluctuations become significant they may set limits to the level of frequency noise suppression. If a Mach-Zehnder interferometer is used as the frequency discriminator, the phase difference can be selected so that intensity fluctuations are suppressed. However, a Mach-Zehnder interferometer with this property will be quite bulky.
As an alternative solution to this problem, a setup according to the one shown in
Thus, starting with
The feedback control means 440 is, in turn, arranged to adjust the output electrical signal from the frequency noise discriminator using the electrical signal received from detection means 432, whereby a disturbance of the output electrical signal from the frequency noise discriminator 430, 431 arising from an intensity change in the light output by the laser 410 is attenuated. One way of achieving this is to subtract, using summing means 444, the value of the detected light power, which has first been equalized using variable gain amplifier 442, to the signal from amplifier 443, as shown in
In this case, a separate control loop that suppresses the intensity noise can be implemented, by feeding back the signal from a light intensity detector 532 to the SOA section 514 of the laser 510 rather than using it to compensate the output from discriminator 530, 531.
Thus, in this case the device 500 comprises light intensity detection means 532 arranged to detect an intensity change in light output from the laser 510, tapped off from beam 516 using one of light splitters 520, and to convert the detected intensity change into a change in an electric signal, which electric signal is used to adjust a current fed to the separate optical amplifier section 514 of the laser 510, whereby the change of the output laser light intensity is counteracted. In
The present invention in its various embodiments is especially applicable as a part of a transmitter or a receiver local oscillator in coherent communication systems. In the former case, the corrected output beam 116 (see
The present invention furthermore relates to a method for reducing linewidth and frequency noise in a conventional DBR laser device. Such a conventional DBR laser device 100, 200, 300, 400, 500 typically comprises a DBR laser 110, 210, 310, 410, 510, in turn comprising at least one Bragg section 111, 211, 311, 411, 511, at least one phase section 112, 212, 312, 412, 512 and at least one gain section 113, 214, 313, 413, 513; a laser control means 150, 250, 350, 450, 550, arranged to feed respective tuning currents to the said at least one Bragg section, to the said at least one phase section and to at least one gain section in order to tune the laser to a certain WDM channel. According to this method, to such a laser device is provided a feedback control means 140, 240, 340, 440, 540 and a frequency noise discriminator 130, 131, 230, 231, 330, 331, 430, 431, 530, 531 of the type described herein. Then, the combined arrangement is operated according to the principles as described herein in order to reduce the linewidth and frequency noise of the laser.
Above, preferred embodiments have been described. However, it is apparent to the skilled person that many modifications may be made to the described embodiments without departing from the idea of the invention.
For instance, the embodiment shown in
Other combinations of feeding individual negative feedback currents to a number of passive sections in a DBR laser are envisaged in a similar manner. All of these embodiments can also freely be combined with the
Furthermore, other types of DBR lasers, as exemplified in the introductory section above, may be used apart from the simple ones used as examples in
Thus, the invention shall not be limited to the described embodiments, but can be varied within the scope of the enclosed claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 7471710 | Cliche et al. | Dec 2008 | B2 |
| 7620081 | Shahine | Nov 2009 | B2 |
| 20060153253 | Diffily et al. | Jul 2006 | A1 |
| 20130243015 | Eriksson et al. | Sep 2013 | A1 |
| Number | Date | Country |
|---|---|---|
| 1051045 | Apr 2012 | SE |
| Number | Date | Country | |
|---|---|---|---|
| 20140177659 A1 | Jun 2014 | US |
