Multi-wavelength Fiber Laser Operating At Room Temperature

Since 1990s, the world has stepped into the era of "information explosion", the scale of global communications has thereby dramatically increased, which has exceeded the full capacity of single-wavelength communication system. As a result, how to enlarge the capacity of the communication system aroused general interest. WDM and OTDM overcome optical components and electronic components bottleneck effect in traditional TDM, therefore they have developed quickly and became the main technologies instead of TDM. At the same time, the development of multi-wavelength erbium-doped fiber lasers (EDFL) has attracted so much research interest because of their potential use in optical communications as the light source.Several methods have been proposed to obtain multi-wavelength operation, such as cooling the EDFA to 77K to reduce the homogeneous line-width, using Raman amplification, Stimulated Brillouin Scattering (SBS) and a semiconductor optical amplifier (SOA) as an additional gain medium inside an EDFL cavity, and adding a frequency shifter to EDFL. In this paper, we present the design of a stable multi-wavelength EDFL, using a PM-fiber Sagnac loop filter and a PZT-based all-fiber phase modulator driven by a sine-wave generator. The phase modulation cause a shift of the lasing frequencies being fed back to the gain medium and result in simultaneous multi-wavelength lasing operation with 0.9 nm wavelength spacing. 980/1550nm WDM, EDF with 8m length, 0.23 NA, 6.39×10²⁴ / m³ erbiumconcentration, a optical isolator(ISO),a PZT-based all-fiber phase modulator driven by a sine-wave generator and a PM-fiber Sagnac loop filter. The PM fiber Sagnac loop filter as a key component is constructed by a PM Coupler, 5m of PMF, and a polarization controller.Fig.1 Experimental setup of multi-wavelength fiber laserThe input light is split into two counter-propagating waves by the PM coupler, and the two waves are recombined at the coupler output after traveling through the loop. The high birefringence fiber supports two linearly orthogonal fundamental modesHE₁₁^x and HE₁₁^y with propagation constantsβ_x(ω) andβ_y(ω) respectively, whereωis the angular frequency of the light. The polarization controller (PC2) which is added at one end of the Sagnac loop filter can cause the rotation of 90°to theprinciple axes. Clockwise wave (CW) and counterclockwise wave (CCW) experience different optical length and thus there is phase difference, and, as a result they, interfere when recombined at the PM coupler. The period of Sagnac filter is fixed when the length of PMF is set to a const, but the wavelength is tunable by adjustinjPCl.The reflectivity of Sagnac loop filter can be easily derived asR = K(41)MJP(θ)K₁₃ + K₃₁MP(θ)JK₁₄where k is the splitting ratio of PM coupler,θis the angle that PC2 adjusted , n_ethe refractive index of extraordinary ray , n₀ is the refractive index of ordinary ray and d is the length of PMF.Numerical Simulation:Fig 2 Effect of length change of PMF on reflective intensity of Sagnac loop filter Fig 3 Effect of birefringence change of PMF on reflective intensity of Sagnac filterFig 4 Effect of splitting ratio change of PM coupler on reflective intensity of Sagnac PMF loop filterFig 5 Effect of angle change of PC2 on reflective intensity of Sagnac PMF loop filter Fig 6 Effect of polarization state change of input light on reflective intensity of Sagnac PMF loop filterIt is noted that, maximal extinction ratio is obtained when k =(?) andθ=(?),and the lasing wavelength and the wavelength spacing depend on the length of PMF and the birefringence of PMF, not the polarization state of input light.The operating principle of the proposed room-temperature multi-wavelength fiber laser with intra-cavity sinusoidal phase modulation is that the phase modulator shifts the light to neighboring frequencies, and the light with a shifted frequency is fed back to the gain medium in every round trip, resulting in a multiple wavelength lasing operation at room temperature. Assume the input light has a cos(ωt -φ₀) form, themodulated light is governed by cos(ωt-φ₀ +φ_m cosω_mt), whereωis the angle frequency of the input light,φ_m anω_m are the modulation amplifier and modulation angle frequency, respectively. Equation can be written asE₀cos(ωt-φ₀)J₀(φ_m)+E₀(?)J_n(φ_m){cos[(ω+nω_m)t-φ₀]+(-1)ⁿcos[(ω-nω_m)t-φ₀]](2)Equation (2) containsω±nω_m frequency components corresponding to the amplitude components J_n(φ_m) of the lasing wavelengths, where J_n(φ_m) are Bessel functions of the first kind of order n (n = 0,1,2…). When J₀(φ_m) = 0, the light frequency will be shifted fromωtoω±nω_m . Experiment Results and DiscussionSince the gain spectrum of an erbium-doped fiber (EDF) is homogeneously broadened at room temperature, multi-wavelength operation is not possible in a simple erbium-doped fiber laser due to cross-gain saturation effects. As a result, without phase modulation the lasing wavelengths show in Fig 7 did show amplitude instability.When a 29kHz, 100mV sine signal was applied to the PZT phase modulator, stable room-temperature operation of the multi-wavelength with 0.9nm wavelength spacing was observed . It was found that when modulation voltage was lower than 60mV multi-wavelength oscillation became unstable again just like the output spectra of the fiber laser without phase modulation. It is because the sinusoidal generator did not apply enough amplitude to the PZT. Fig 8 Output spectra of the fiber laser with phase modulationWhen the modulation frequency had been taken from 10-50 kHz, the equation J₀(φ_m)= 0 was always satisfied, so the multi-wavelength lasing operates at room temperature.Analysis1 The fact that there is a minimum threshold voltage of periodic signal can be theoretically explained by equation (2), in which J₀(φ_m) is the amplitude offrequencyω, withφ_m being direct proportional to the voltage periodic signal. Becausethe 0^th order Bessel function J₀(x) is an oscillation decreasing function, its value can be deemed as 0 when the voltage has exceeded a certain value.2 When the voltage is fixed at 100mV and the frequency is adjusted between 27.5 and 30.2 kHz, multi-wavelength output can be obtained. When it is above 30.2 kHz or below 27.5 kHz the experimental results is identical to those obtained without a frequency shifter. The reason for such a range of frequencies is that mode-locking is not required for multi-wavelength output. Problems and DiscussionIt can be noted that, though the multi-wavelength output position is stable, there are still problems such as low laser power, low number of multi-wavelength output and output power not being as stable as theoretically simulated.1 Low laser power and low number of multi-wavelength output. The reason lies in the fact that the power of spontaneous emission generated by pump light through EDF is relatively low, which not only directly leads to the low power of output laser, but, more importantly, is not conducive to producing more multi-wavelength output. Both the power of spontaneous emission and central wavelength of pump source matching with the selected erbium-doped fibers or not is the factor which can affect the experiment output. Consequently, we should have the peak wavelength of pump source as fitting to the absorbance peak wavelength of the selected erbium-doped fibers as possible. In this experiment, the peak wavelength of the 980nm pump source is 976nm, and the absorbance peak wavelength of the erbium-doped fiber is around 978nm. Besides, the choice of erbium-doped fiber length is critical as well.Fig 4-9 ESA with different length of EDF According to the curve showing the theoretically simulated relationship between the spontaneous emission and fiber length based on specific parameters of EDF in the experiment, the appropriate length of the erbium-doped fiber should be around 3m, with 2.8m as the theoretically optimal one.2 The instability of peak power may result from the disturbance of the polarization state in fiber loop mirrors. Although the polarization state of input light has no influence on filter properties of the loop filter of Sagnac polarization-maintaining fiber, in theoretically simulation, the polarization state of input light is found to be a complex function of various parameters including polarization state and wavelength of output light. Due to the different degrees of polarization loss, the power of light varies with its polarization state. The fibers adopted in the experiment are not exactly polarization-maintaining fiber, in which the phase modulator is also comprised of single-mode fibers, so there can be disturbance of the polarization state, which can affects peak power. Therefore, as is permitted by experiment conditions, polarization-maintaining fiber ought to be used to replace SM fiber.ConclusionThis paper presents a thorough exposition on the filtering principles of the loop structure of polarization-maintaining fibers, discusses the influences of various parameters on its filter properties, and proposes an experimental scheme of multi-wavelength fiber laser. Accordingly, when sine signal and square wave signal are applied to the phase modulator, a multi-wavelength output with five peaks with a 0.9nm interval was obtained at room temperature. Finally, we probe into the existing problems and possible solutions to them.