H. Halbritter, F. Riemenschneider, B. Ko¨gel, E. Feldmeier and P. Meissner A novel idea and realisation of a control algorithm to switch the wavelength of continuously tunable lasers is presented. The concept allows the switching of a defined number of dense wavelength division multiplexed (DWDM) channels up or down by employing a broadband wavelength locker. The switching algorithm is implemented and demonstrated for a micro-electro-mechanically tunable verticalcavity surface emitting laser (MEM–VCSEL).
Introduction: Continuously tunable lasers are gaining increased interest. In addition to applications in gas sensing, a strong focus lies on the employment in dense wavelength division multiplexing (DWDM) systems. In these communication systems several different, fixed wavelength lasers are operated today. In next generation systems increased flexibility and functionality of the system and thus of the components is required. These enhanced criteria can be fulfilled by e.g. the use of tunable lasers, which can be operated over a broad range of wavelengths, either to increase the flexibility of networks or to serve as a backup for several fixed wavelength lasers. Furthermore these lasers need to be tuned and locked to any arbitrary wavelengths defined by the International Telecommunication Union (ITU). In traditional wavelength switching concepts employed for widelytunable lasers, e.g. sample grating distributed Bragg reflector lasers (SG-DBR), this is usually done with memory based look-up tables and microcontrollers. The look-up information requires extensive and thus costly testing and data evaluation of the laser wavelength and tuning characteristics and dependencies [1]. Additionally, longterm data collection and evaluation of the lifetime and temperature behaviour of the emitted wavelength against control parameters is mandatory to avoid drift related wavelength (parameter) deviations. Micro-electro-mechanically continuously tunable lasers, e.g. vertical cavity surface emitting lasers (MEM–VCSELs), exhibit modehopping free, continuous tuning and allow subsequently the employment of novel and simple algorithms for channel switching. In this Letter a concept for deterministic wavelength switching and its realisation is demonstrated. The setup includes a broadband standard wavelength locker and the implementation of the algorithm within simple analogue control circuitry.
Fig. 1 Principle schematic of wavelength locker currents a Scheme for switching to higher frequencies (lower wavelength) b Scheme for switching to lower frequencies (higher wavelength) The numbers indicate the steps in the switching routine according to Table 1
Concept: A broadband Fabry-Pe´rot wavelength locker, as employed in this switching algorithm, comprises a reference pin-photodiode and an etalon-coupled pin-photodiode. Wavelength locking is achieved when the reference photocurrent Iref and the etalon photocurrent Ietalon are equal (Iref ¼ Ietalon) and dIetalon=df > 0 (standard lockpoint at the positive slope, see Fig. 1). Simple control circuitry with feedback usually locks the wavelength on the standard lockpoint (see Fig. 2). Such basic analogue circuitry consists of a logarithmic amplifier, a lowpass filter to prevent any oscillation, and a voltage-current converter (if the wavelength tuning is current driven). The realisation of an inverting or noninverting feedback structure depends on the wavelength against the tuning parameter dependency of the tunable laser. By considering a strictly monotonic relationship between the tuning parameter, typically current or voltage, and the emission wavelength l0 of a tunable laser [2, 3], the proposed novel switching scheme can be implemented. Introducing an artificial offset in one of
the two photocurrents of the wavelength locker allows us to shift the lockpoint. This results in an emitted wavelength, which is slightly decreased or increased from the standard lockpoint wavelength (see switch A in Fig. 2). By inverting the feedback loop the lockpoint can be switched to the opposite slope of the etalon photocurrent (see switch B in Fig. 2). Combining these manipulations in the right order leads to a defined and deterministic tuning of the wavelength towards the next higher or lower standard lockpoint. Table 1 presents the necessary manipulations and their timings to switch the laser wavelength to the adjacent lockpoint or wavelength.
Fig. 2 Block diagram of wavelength switching control electronics
Table 1: Wavelength (channel) switching procedure 1 laser locked at frequency f0 (wavelength l0) (switch A ¼ 0, switch B ¼ 1) 2
add offset (switch A ¼ 1) (add offset to Iref)
add offset (switch A ¼ 2) (add offset to Ietalon)
3
lock at falling slope (switch B ¼ 1) and remove offset (switch A ¼ 0)
lock at falling slope (switch B ¼ 1) and remove offset (switch A ¼ 0)
4
add offset (switch A ¼ 2) (add offset to Ietalon)
add offset (switch A ¼ 1) (add offset to Iref)
5
lock at rising slope (switch B ¼ 1) and remove offset (switch A ¼ 0)
lock at rising slope (switch B ¼ 1) and remove offset (switch A ¼ 0)
6
laser locked to frequency f0 þ 100 GHz (wavelength l0 Dl)
laser locked to frequency f0–100 GHz (wavelength l0 þ Dl)
Realisation: A tunable MEM–VCSEL according to [3], with a strictly monotonic wavelength dependency on the dissipated actuation power inside the tunable membrane (Dl DI2tuning, dl=dItuning > 0), was employed in the experiment. Further a broadband Fabry-Pe´rot wavelength locker with 100 GHz spacing was used. The necessary photocurrent offset was created by a 10:1 pin-photodiode current mirror, adding 20% offset (common cathode wiring of the wavelength locker) to either the reference or etalon photocurrent (see Fig. 2). This results subsequently in a wavelength shift towards longer or shorter wavelengths. A standard relay realises this function and prevents any leakage current (see Fig. 2, switch A). The slope selection is done with either an inverting or noninverting amplifier structure in the feedback loop (switching between the slopes was realised by a relay, see Fig. 2, switch B). 187.4 -40
rel. optical power, dB
Optical channel switching algorithm for continuously tunable lasers
5
frequency, THz 187.35 4
3
187.3 2
1
187.3
frequency, THz 187.25
1
3
2
4
187.2 5
-50 -60 -70 100 GHz
-80
100 GHz
-90 1599.8 1600.0 1600.2 1600.4 1600.6 wavelength, nm a
1600.6 1600.8 1601.0 1601.2 1601.4 wavelength, nm b
Fig. 3 Optical spectrum analyser graphs of switching process a Towards lower wavelengths (from 1600.60 to 1599.75 nm) b Towards higher wavelengths (from 1600.60 to 1601.46 nm) The numbers refer to the actual step in the switching routine (refer to Fig. 1 and Table 1)
ELECTRONICS LETTERS 25th November 2004 Vol. 40 No. 24
The implemented switching procedure worked over the complete tuning range of the MEM–VCSEL, down to a wavelength locker optical input power of 40 dBm. By recording the switching history the absolute or relative wavelength is known at any time. Fig. 3 presents the recorded optical spectra of the laser, before and after wavelength switching to the adjacent wavelength (Df ¼ 100 GHz). Included in the graphs are the spectra after execution of every step of the procedure, as listed in Table 1. Fig. 4 presents the measured lockpoint error voltage, 1 V log(Ietalon=Iref) (refer to the block diagram in Fig. 2), representing the wavelength deviation from the lockpoint. The switching speed is limited by the 3 dB bandwidth of the tunable membrane, which is around 100 Hz for the employed membrane structure. Based on this limitation a complete channel switching was realised within 50 ms, either to the adjacent higher or lower wavelength channel. This corresponds well with the expected switching speed, as four membrane tuning steps are necessary for executing the complete wavelength change. If a fast switching time is desired, the lockpoint error voltage does not need to be settled and zero before the next step is initiated, as demonstrated in Fig. 4. By employing faster membrane structures (i.e. optimised design or electrostatic actuation of the membranes) the switching time can be reduced considerably.
Conclusions: For the first time a novel mechanism for deterministic wavelength hopping from one channel to the next is presented. The scheme can be employed with any tunable laser which features a strictly monotonic function of the emitted wavelength against the control parameter, as demonstrated with a tunable MEM–VCSEL. Reliable wavelength switching has been demonstrated with a standard broadband wavelength locker. The wavelength switching scheme has been successfully implemented in a simple analogue control circuit. Acknowledgment: This work has been supported by German BMBF research project 01BP271. # IEE 2004 Electronics Letters online no: 20047039 doi: 10.1049/el:20047039
17 September 2004
H. Halbritter, F. Riemenschneider, B. Ko¨gel, E. Feldmeier and P. Meissner (Technische Universita¨ t Darmstadt, Institut fu¨ r Hochfrequenztechnik, Merckstr. 25, 64283 Darmstadt, Germany) E-mail:
[email protected] References 1
2 3
O’Dowd, R., O’Duill, S., Mulvihill, G., O’Gorman, N., and Yu, Y.: ‘Frequency plan and wavelength switching limits for widely tunable semiconductor transmitters’, IEEE J. Sel. Top. Quantum Electron., 2001, 7, (2), pp. 259–269 Chang-Hasnain, C.J.: ‘Tunable VCSEL’, IEEE J. Sel. Top. Quantum Electron., 2000, 6, (6), pp. 978–987 Riemenschneider, F., Sagnes, I., Bo¨hm, G., Halbritter, H., Maute, M., Symonds, C., Amann, M.-C., and Meissner, P.: ‘A new concept for tunable long wavelength VCSEL’, Opt. Commun., 2003, 222, pp. 341–350
Fig. 4 Switching time measurement (lockpoint error voltage, refer to the block diagram in Fig. 2) a Towards lower wavelengths b Towards higher wavelengths The numbers refer to the actual step in the switching routine (refer to Fig. 1 and Table 1)
ELECTRONICS LETTERS 25th November 2004 Vol. 40 No. 24
