OFSDON

 

Optical Frequency Synthesis for
DWDM Optical Networks

C.C. Renaud, C.F C. Silva and A.J. Seeds

  

DWDM : Dense Wavelength Division Multiplex

Introduction

Optical injection phase lock loops are utilized for wavelength locking widely tuneable lasers to an absolute referenced optical frequency comb. Locking is maintained under 5oC laser temperature variation.

For full utilization of the installed base of fibre links with conventional EDFAs and NRZ intensity modulation/detection schemes, high spectral efficiency in ultra-dense WDM architectures is required. Non polarization interleaved systems with 0.4 bit/s/Hz (25 GHz channel spacing at 10 Gbit/s) and 0.53 bit/s/Hz (18.8 GHz channel spacing at 10 Gbit/s) have been demonstrated [1]. With such narrow channel spacing, wavelength stabilization of the channel lasers is crucial to achieving reliable operation with the reduced guard bands. Also, the use of widely tuneable lasers is of great interest for reducing inventories, implementation of dynamic wavelength provisioning and simplification of network control software.

Conventional DWDM systems use precision temperature control of the lasers together with locking of the laser wavelength to an external optical reference filter such as a Fabry-Perot etalon, AWG or Mach-Zehnder, giving stability of the order of ±1 GHz [2, 3]. These reference filters suffer from calibration offsets, aging and temperature drifts and have themselves to be temperature stabilized. A more precise method uses absorption lines of atoms or molecules as references. In particular, 20 kHz stability has been reported [4] using an atomic Krypton line. The main drawbacks of this technique are the unavailability of equally spaced absorption lines, cost and dimensions of the absorption cell.

An attractive way to overcome these limitations is to use the emission of a single precision stabilized laser to generate a comb of equally spaced master optical lines, and lock a number of slave lasers to different master comb lines by optical injection locking (OIL). In [5] we demonstrated the approach using a fibre loop comb generator and a widely tuneable sampled grating DBR (SG-DBR) laser diode. However, the locking condition could only be maintained for a limited range of slave laser frequency drift, around 500 MHz. In this paper, we present results for a new locking scheme, which uses the combination of OIL and optical phase lock loop (OPLL) techniques in a so called optical injection phase lock loop (OIPLL) method, which can maintain lock over a much wider range (~ 80 GHz), enabling reliable locking of the laser diode, with aging and temperature variations.

 

Locking system

The locking system is shown in Figure 1, where the OIL and OPLL parts of the system are highlighted. The butterfly packaged SG-DBR laser, specially assembled without an internal optical isolator, is connected to port 2 of an optical circulator. From port 1 the signal from a comb generator travels towards the SG-DBR laser. The fibre loop comb generator used [5] was driven by an external cavity reference laser and a microwave reference synthesizer, set to 18 GHz, responsible for exact setting of the comb frequency spacing. Travelling from the SG-DBR laser towards the circulator there are two signals. One is the strong emission from the SG-DBR laser diode and the other is a weak reflection of the incident comb from the anti-reflection coated facet of the laser. The reflected comb has average power 25 dB lower than the incident comb signal. Both signals travel from circulator port 2 to port 3 towards the output of the system. Figure 2(a) shows the measured injected comb that travels towards the laser and the output from port 3 of the circulator. A minimum unwanted comb line rejection of 40 dB is observed. The initial values of the SG-DBR currents for different channels were stored in a look-up table in the computer used to control the current sources and the temperature controller.

 

 

This comprises the OIL part of the system. With the OPLL control circuit switched off (VC = 0), a measured locking range of 500 MHz, when detuning the slave laser, was observed using a scanning Fabry-Perot analyzer connected to the system output. This is as expected for the -30 dB injection ratio for the chosen comb line [5].

For the OPLL part of the system, 10% of the output power is tapped off and sent to the OPLL control circuit. The photo-detector generates a microwave signal with frequency components at the comb spacing and its multiples. As only the lowest frequency component is to be used by the OPLL circuit, the photo-detector bandwidth needs only to equal the microwave reference frequency, 18 GHz in our case. This signal, generated by the heterodyne between the slave laser emission and the residual comb reflected from the slave laser facet, is then amplified and sent to the RF port of a microwave balanced mixer.  Working as a phase comparator, the balanced mixer IF port outputs a DC signal, from mixing the signal at the RF port with the original signal from the microwave reference. Slave laser frequency variations are converted in this way to an error signal which, after integration and amplification in the loop filter, feeds a control voltage VC back to the current source driving the phase or gain section of the SG-DBR laser. The OIPLL circuit is a 2nd order type II loop, and its phase error is dominated by the phase noise of master source and slave laser. A reduced phase error implies an improved phase tracking characteristic of the slave laser to the master source.

 

 

Fig. 2. (a) Spectra of locked slave laser emission and injected reference comb. (b) Emited wavelength of locked and free runnning slave laser with laser submount temperature detuning. Output power and SMSR in locking condition.

 

Results

 

Figure 2(b) shows the measured locking range. For these measurements, the control voltage VC generated by the loop filter was applied to the SG-DBR gain section, for convenience. The temperature of the SG-DBR laser was swept using the computer and its output wavelength measured using an Agilent 86142B optical spectrum analyser. The laser is seen to maintain lock over a 5 oC temperature rage. For comparison, the figure also shows the free running temperature tuning characteristic of the laser, with 0.121 nm/oC slope.

The peak output power and the side mode suppression ratio (SMSR) of the SG-DBR laser output are also plotted in Figure 2(b). The locked line power stability was better than 3 dB and the SMSR was > 35 dB. During the temperature tuning, the VC excursion was 400 mV, altering the gain section current of the SG-DBR laser between 87 mA and 127 mA. This is responsible for the output power variation. Application of VC to the phase section instead can eliminate this effect. The locking range was limited by the relative detuning of the front and rear reflectors of the SG-DBR laser. This range can be increased further by applying control signal to the reflector sections, ensuring mode stability [2, 3].

 

References

[1] V. Mikhailov, P. Bayvel and E. G. Churin, “0.53 bit/s/Hz Spectral Efficiency Free-Space Demultiplexer With Very Low Crosstalk, Loss and Polarisation Dependence”, Electron. Lett. 36, 1640-1641 (2000).

[2] G. Sarlet, G. Morthier and R. Baets, “Control of Widely Tunable SSG-DBR Lasers for Dense Wavelength Division Multiplexing”, J. of Lightwave Technol. 18, 1128-1138 (2000).

 

[3] H. Ishii, F. Kano, Y. Yoshikuni and H. Yasaka, “Mode Stabilization Method for Superstructure-Grating DBR Lasers”, J. of Lightwave Technol. 16, 433-442 (1998).

 

[4] C.R. Schwarze and J.H. Rentz, “Frequency Stabilized Distributed Feedback Laser Diode System at 1323 µm Using the Modulated Zeeman Effect”, Rev. Scientific Instruments, 70, no. 10, pp. 3828-3831 (1999).

 

[5] C.F.C. Silva, A.J. Seeds and P.J. Williams, “Terahertz Span > 60 Channel Exact Frequency Dense WDM Source Using Comb Generation and SG-DBR Injection-Locked Laser Filtering”, 13, 370-372 (2001).