Design of External Cavity Semiconductor Lasers to Suppress Wavelength Shift and Mode Hopping

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Ariel York
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1 ST/03/055/PM Design o External Cavity Semiconductor Lasers to Suppress Wavelength Shit and Mode Hopping L. Zhao and Z. P. Fang Abstract In this report, a model o ernal cavity semiconductor laser is built, and then based on this model a program is coded or simulating the thermally induced wavelength shit and mode hopping. By comparing the simulation and the published experimental results o two modiied ECL designs we ound that our simulation results agree very well with the experimental results. Through theoretical analysis and simulation, it is ound or the irst time that the thermally induced mode hopping will be increased by using temperature insensitive Bragg grating in WG-ECL. Furthermore, a novel design o ernal cavity semiconductor lasers with suppressed thermally induced wavelength shit and mode hopping is proposed. The calculation results show that the wavelength stability with temperature is dramatically improved and the side mode suppression ratio (SMS) remains higher (>35 db) when temperature changes rom 0 o C to 50 o C. Keywords: External cavity semiconductor laser (ECL), Temperature insensitive iber Bragg grating (), Mode hopping, Wavelength shit BACKGOUND The ernal cavity semiconductor laser (ECL) is a promising light source or the uture dense wavelength division multiplexing (DWDM) systems and access area network. Waveguide grating ernal cavity semiconductor laser (WG- ECL) or iber grating ernal cavity semiconductor laser (FG-ECL) is composed o a UV written Bragg grating on a silica waveguide or a iber, and a laser diode (LD) with high relection (H) coating on one acet and anti relection(a) coating on the other acet. The oscillation wavelength o WG-ECL is less sensitive to temperature than that o the conventional distributed eedback lasers due to the relatively low temperature coicient o the silica waveguide. However, thermally induced mode hopping in WG-ECL or FG-ECL has to be suppressed because it will aect the stability o output power and side mode suppression ratio (SMS). The channel spacing o uture DWDM system will be less than 0.8 nm and the tolerance o emitted wavelength should be less than 0% o the channel spacing on the whole operating range. Consequently, thermally induced wavelength shit in WG-ECL or FG-ECL (normally. nm/00 o C) has to be urther reduced or the uture DWDM applications. Experimental study has shown that the thermally induced wavelength shit can be reduced in ECL by using temperature insensitive Bragg grating [], but there is no study on the thermally induced mode hopping in ECL when using temperature insensitive Bragg grating. On the other hand, although it has been reported that thermally induced mode hopping in ECL can be suppressed by using a silicone wave-guide with negative thermo-optic coicient as a part o the ernal cavity [], there is no improvement on the thermally induced wavelength shit using this structure. OBJECTIVE In this report, we set a model o ECL and develop a simulation program to calculate the thermally induced wavelength shit and mode hopping. We make the simulation o the two published modiied ECL designs, and compare the simulation results we got and the published experimental results in order to prove that the model and the simulation are correct. Through our simulation and analysis we propose a novel design o WG-ECL to suppress simultaneously both the thermally induced wavelength shit and the thermally induced mode hopping. 3 METHODOLOGY 3. Theoretical study o thermally induced mode hopping and wavelength shit in ECL Thermally induced mode hopping in WG-ECL is mainly due to the act that the ernal cavity modes shit more rapidly than the Bragg wavelength o grating with temperature change. Hence, the temperature interval o mode hopping t is given as: λ t = dλc / dt dλb / dt ()

2 where τ is related to the cavity mode spacing, λ, the thermal coicient o cavity mode, dλ c /dt, and the thermal coicient o Bragg grating, dλ B /dt. (A coating) Pront On the other hand, thermally induced wavelength shit in WG-ECL or FG-ECL is determined by the thermally induced Bragg wavelength shit o Bragg grating, which is given by: dλ dt = λb n dn dt B dλ + Λ dt () where λ B is the Bragg wavelength, Λ and n are the period and ective reractive index o the Bragg grating, respectively. From () and (), the condition o no thermally induced mode hopping and no thermally induced oscillation wavelength shit is given by: dλ / dt = dλ / dt = 0 (3) c B In the ollowing calculations, the.55 µm InP QWs LD with H coating (=95%) and A coating (=0-4 ) is modeled using multimode rate equations [3]. The variation o peak optical gain with temperature is also taken into account using an empirical ormula and the corresponding optical gain spectrum is approximated by a parabolic curve. The output power is maintained around 3 mw. 3. Model o FG-ECLs or light source Fig. (a) shows a schematic model o Fiber Grating ernal cavity semiconductor Lasers (FG-ECLs) used in the report. is the intensity relectivity o one end o laser diode with H coating, is the intensity relectivity o the other end o the laser diode with A coating. As shown in Fig. (b), the iber grating is replaced by an equivalent mirror with an intensity relectivity coicient ( ) and a phase delay (Φ ). Here both the phase delay (Φ ) and relectivity ( ) induced by the relection o iber grating in terms o wavelength are considered. The equivalent mirror is assumed to be located at the start point o the iber grating. In the calculation, we assume = 0. Fig.. Model o FG-ECLs or light source. The intensity relectivity coicient ( ) is given as: = η (4) Here η is the laser-to-iber coupling iciency. The phase delay (Φ ) is given [4] as: φ φ = (5) As shown in Fig. (c), the ects o multiple relections in the ernal cavity can be represented by an ective incoherent relectivity, which is given as: ( ) ( λ ) = + η (6) When = 0, the ective intensity relectivity coicient (λ) can be simpliied as: = (7) T (a) Schematic coniguration o ECL LD LD (c) Equivalent coniguration o ECL. η( ) = (8) L (b) Equivalent coniguration o ECL T The threshold gain condition is changed due to the optical eedback, which is given as:

3 g th = α int + L D ln Γ (9) The phase condition o ernal cavity mode can be simpliied as: φ + φ + φ = mπ (0) c gap Here φ λ 4πn λ D c ( ) = LD is the phase delay due to the round trip propagation in the laser diode; φ gap 4πn gap = L gap is the phase de- λ lay due to the round trip propagation in the gap between LD and grating. 3.3 electivity and phase delay o the iber gratings (λ) is the relectivity spectrum o Fiber grating. For common FG-ECLs, the relectivity is about 40%-60%, in this case, ) can be simpliied as [5]: (λ G 0 =.nm cm / γ =0-4 w =3µm d=0.µm L=300µm C d = α d = External cavity parabolic gain itting actor spontaneous emission actor width o the active region thickness o the active region length o laser diode thermo-optic coicient o LD thermo-expansion coicient o LD η =0.5 coupling iciency n =.495 ective reractive index o gratings λ B =550nm Bragg wavelength C B =7 0-6 / o C thermo-optic coicient o silica α B =5 0-7o C thermo-expansion coicient o silica λ =0.5nm relection bandwidth o Bragg grating Lg=50cm length o gap max =0. Maximum relectivity o Grating 3.5 Simulation o the two modiied ECLs ( λ λ [ λ ) ] = max max () B Phase delay due to the relection o φ (λ) can be simpliied as [5]: φ π π = + ( λmax λ) λ () B 3.4 Parameters used in simulation Based on the multi-mode rate equation [6-8], we developed the simulation program and the simulations were conducted. I not separately stated, the parameters used in the simulation are as ollows Laser diode α int =40 cm - internal loss in laser diode µ g =3.5 group reractive index o semiconductor material τ s =ns spontaneous emission (carrier) lietime n 0 = cm -3 carrier density to reach zero gain Г =0.5 coninement actor A= cm linear gain constant Fig.. Simulation o the modiied FG-ECL design proposed by T. Tanaka to suppress the thermally induced mode hopping. First, we simulated the modiied FG-ECL design proposed by T. Tanaka [] to reduce the thermally induced mode hopping. In that design a speciic material silicone, which has negative thermo-optic coicient, is used as part o the ernal cavity to reduce the whole ect o the thermo-optic coicient o the cavity. Our simulation result as shown in Fig. agrees very well with the published experimental results. We can observe that by using this structure, the thermally induced mode hopping will coincide with the Bragg wavelength shit o the 3

4 grating and suppress the mode hopping. However there is not any improvement in the thermally induced wavelength shit. Fig. 3. Simulation o the modiied WG-ECL proposed by Leroy to reduce the thermal induced wavelength shit. We also simulated another modiied WG-ECL design with temperature insensitive iber Bragg grating proposed by Leroy [] to reduce the thermal induced wavelength shit. This design uses a speciic polymer to orm the temperature insensitive integrated wave-guide grating. From our simulation results as shown in Fig. 3, which agree well with the experimental results, we can see that the thermally induced wavelength shit is reduced dramatically. However, the mode hopping is increased because that by using temperature insensitive grating, the thermally induced cavity mode shit is much aster than the Bragg wavelength shit. By comparing the simulation and the published experimental results o the two modiied ECL designs we ound that our simulation results agree very well with the experimental results, and this relects that we have built a good model to simulate ECL. in section 3 it is ound that the thermally induced wavelength shit is reduced due to the use o temperature insensitive Bragg grating. However, the temperature interval o thermally induced mode hopping is also reduced because the dierence between dλ B /dt and dλ c /dt is increased by the use o temperature insensitive grating, Thereore there is much more mode hopping or a certain mount o temperature change. In order to suppress both the thermally induced mode hopping and the thermally induced wavelength shit, a novel structure o integrated WG- ECL is proposed as shown in Fig. 4. In Fig. 4, core and cladding regions o the wave-guide are constructed using SiO and SiO +GeO. A 5mm wave-guide gap including a 300 µm silicone (dn g /dt= / o C) and 4.7 mm silica is used to suppress thermally induced mode hopping. A polymer [6] with negative thermo-optic coicient (dn /dt = / o C) is used as waveguide cladding to decrease the temperature sensitivity o Bragg grating (λ B /dt nm/ o C). (a) 4 ESULTS & DISCUSSION A 300um Silicone SiO +GeO H LD Polymer SiO SiO SiO Si SiO LD=300um Lg=5mm LwG=0mm Fig. 4. Proposed structure o WG-ECL to suppress both thermally induced mode hopping and wavelength shit. Through the simulation and theoretical analysis (b) Fig. 5. Calculated wavelength stability with respect to temperature in (a) ECL with common uniorm iber Bragg grating; (b) proposed WG-ECL. Fig. 5 is the calculated wavelength stability with temperature. Compared with Fig. 5(a), it is shown in Fig. 5(b) that there is no thermally induced mode hopping in a temperature range o 30 o C (0 o C~50 o C) in our proposed structure. The thermally induced wavelength shit is also 4

5 as small as dλ/dt = 0.007nm/ o C due to the use o temperature insensitive Bragg grating. Consequently, the thermally induced mode hopping and the thermally induced wavelength shit are both suppressed in our proposed WG-ECL. shit and thermally induced mode hopping by using temperature insensitive Bragg grating and a suitable length o gap with a negative thermooptic coicient. It is shown theoretically that there is no thermally induced mode hopping over a temperature change o 30 o C and thermally induced wavelength shit is also as small as nm/ o C by using the proposed structure. 6 INDUSTIAL SIGNIFICANCE The technology o design and simulation o ECL can be applied to: ) design and development stabilized 980 nm ernal cavity pump laser; (a) ) achieve highly stabilized source lasers or DWDM (with very low thermally induced wavelength shit and mode hopping) 3) design and develop ernal cavity tunable lasers; 4) simulate and design other ernal cavity lasers. This technology is very important and meaningul or the industry o manuacturing ernal cavity lasers and wavelength stabilization, because it can be used to enhance the quality o lasers and explore new products. (b) Fig. 6. Calculated SMS with respect to temperature in (a) ECL with common uniorm iber Bragg grating; (b) proposed WG-ECL. The mode hopping in the common uniorm iber grating ECL can induce the mode competition and then reduce the SMS as shown in Fig. 6(a), we ind that by using this proposed structure, the SMS keeps above 35 db while temperature changes as shown in Fig. 6(b). This relects that the thermally induced mode hopping is suppressed. 5 CONCLUSIONS In this technical report it is ound that the thermally induced mode hopping occurs more requently although the thermally induced wavelength shit can be reduced by using temperature insensitive Bragg grating in WG-ECL or FG- ECL. A novel structure o WG-ECL is proposed to suppress both thermally induced wavelength EFEENCES [] A. Leroy et al., Low-cost wavelength stabilized plug and play lasers or WDM systems in uture local networks, Electronics Letters, Vol. 37(6), pp. 0-04, (00). [] T. Tanaka et. al., Hybrid integrated ernal cavity laser without temperature dependent mode hopping, Electronics Letters, Vol. 35(), pp , (999). [3] T.P. Lee et al., Short-Cavity InGaAsP injection lasers: dependence o mode spectra and single-longitudinal-mode power on cavity length, IEEE Journal o Quantum electronics, Vol. QE-8(7), pp. 0-, (98). [4] A. Olsson and C.L. Tang, Coherent optical intererence ects in ernal-cavity semiconductor lasers, IEEE J. Quantum Electron., Vol. 7(8), pp , (98) [5] F.N. Timoeev et. al., Experimental and theoretical study o high temperature- 5

6 stability and low-chirp.55 um semiconductor laser with an ernal iber grating, Fiber and Integrated Optics, Vol. 9, pp , (000). [6] K. Kallimani and M.J. O Mahony, Calculation o optical power emitted rom a iber grating laser, IEE Proc.-Optoelectron., Vol. 45(6), pp , (998). [7] A.E. Siegman, Lasers, University Science Books Sausalito, Caliornia, (986). [8] M.C. Amann and J. Buus, Tunable laser diodes, Artech House, Boston, (998). [9] D. Bosc et.al., Temperature and polarisation insensitive Bragg gratings realised on silica waveguide on silicon, Electronics Letters, Vol. 33(), pp , (997). 6

Lecture 6 Fiber Optical Communication Lecture 6, Slide 1

Lecture 6 Fiber Optical Communication Lecture 6, Slide 1 Lecture 6 Optical transmitters Photon processes in light matter interaction Lasers Lasing conditions The rate equations CW operation Modulation response Noise Light emitting diodes (LED) Power Modulation