TUNABLE OPTICAL OSCILLATOR BASED ON A DFB-MQW LASER AND A FIBRE LOOP REFLECTOR
Orlando Frazão 1
1Unidade de Optoelectronica e Sistemas Electrónica, INESC PORTO
Rua Campo Alegre, 687, 4169-007 Porto, Portugal
Tel: +351 2 6082601, Fax: +351 2 6082799, Email: firstname.lastname@example.org
Pedro Tavares2, José Ferreira da Rocha 2,3
2 Instituto de Telecomunicações - Polo de Aveiro
3Departamento de Electrónica e Telecomunicações – Universidade de Aveiro
Campus Universitário de Santiago, 3810 Aveiro, Portugal
Tel: +351 34 377900, Fax: +351 34 377901
Luis Botelho Ribeiro 4
3 Departamento Electrónica e Industrial, Universidade do Minho
Tel: +351 53 510190, Fax: +351 53 510189
In this paper we report experimental results on the frequency control of an optical oscillator based on a single cavity DFB-MQW laser and a fiber loop reflector. Self-pulsation frequency can be controlled by changing the step amplitude of the laser bias current.
The use of self-pulsating laser diodes is becoming very attractive in the all-optical networks context. These devices are being used in important sub-systems like all-optical clock recovery and 3R regeneration modules.
Several techniques have been proposed to achieve self-pulsation behavior on a semiconductor laser, but all of them are based on the changing of the cavity characteristics. The simplest method uses gain switching where the cavity gain is modulated using a radio frequency (RF) source . In two or more sections lasers, the Q-switching technique is normally implemented either in a passive or active form . Another possible method to obtain self-pulsation is based on cavity mode locking which can be done again passively or actively . In multi-sections DFB lasers the self-pulsation behavior is due to a new mechanism called dispersive self Q-switching .
This letter reports recent laboratory results of an optical oscillator, where the self-pulsation frequency can be controlled changing the step amplitude of the laser bias current. The physical process responsible for the laser self-pulsation is the injection locking mechanism .
2. EXPERIMENTAL RESULTS
The experimental set-up (see Fig. 1) uses a Multiple Quantum-Well (MQW) distributed feedback (DFB) semiconductor laser. This device operates at 1310nm, having an active length of 250m m and a threshold current of Ith=11.2 mA. One facet is anti-reflection (AR) coated and the other has 30% reflectivity. The light is collected from the laser by a lensed fiber tapper. The AR coated facet of the laser chip is optically coupled to a single mode fibre through a ferrule. In this coupling process some light is reflected from the ferrule and injected back into the laser cavity. The fiber loop, on the ferrule side, was inserted to provide maximum optical coupling . It was verified that the device is very sensitive to the polarization state of the light reflected back from the loop.
Fig. 1 - Experimental Set-up
After setting the distance between the AR laser facet and the fiber ferrule for achieving maximum optical coupling, the frequency of self pulsation can be modified simply by adjusting the bias current (see Fig. 2). For lower currents it was verified that the self pulsation frequency is near 6GHz and the device is stable. For higher currents the self pulsation frequency increases to near 11GHz.
Fig. 2 - Pulsation frequency as function of the bias current
The RF spectrum of the self pulsating signal, for different values of the bias current, is shown in Fig. 3. It can be seen that for 30mA the frequency is approximately 5GHz and for 90mA it goes near 10GHz. It can also be seen the second harmonic near 10GHz for 30mA and near 20GHz for 90mA.
Fig. 3 - RF spectrum for different bias currents.
A experimental set-up configuration was designed and implemented. With this configuration the frequency of self pulsation can be modified by simply adjusting the bias current pulse amplitude between 25 and 100mA. For lower currents it was verified that the self pulsation frequency is near 6GHz and the device is stable. For higher currents the self pulsation frequency increases to near 11GHz.
We would like to thanks Uniphase Netherlands for the semiconductor laser used in this work. This work was sponsored by the UPGRADE project, from the European Union program ACTS (Advanced Communication Technologies and Services) program.