基于PLC的外文论文

DOI:10.1007/s00340-007-2600-3
Appl.Phys.B 87,293–296(2007)Lasers and Optics
Applied Physics B
k.s.lee 1,u c.s.kim 1r.k.kim 1
g.patterson 2m.kolesik 2j.v.moloney 2
n.peyghambarian 2
Dual-wavelength external cavity laser with a sampled grating formed in a silica PLC
waveguide for terahertz beat signal generation
1
School of Information and Communication Engineering,SungKyunKwan University,Suwon 440-746,Korea
2College of Optical Sciences,The University of Arizona,Tucson,AZ 85721,USA
Received:2September 2006/Revised version:9January 2007Published online:22March 2007•©Springer-Verlag 2007
ABSTRACT We propose and demonstrate a dual-wavelength ex-
ternal cavity laser (ECL).In our design,a Fabry–P´e rot laser diode (FP-LD)is hybrid-integrated with a sampled Bragg grat-ing written in a silica planar lightwave circuit (PLC).The grating selects two laser wavelengths that share the same laser cavity.The dual-wavelength oscillation with a side mode sup-pression ratio >32dB has been demonstrated experimentally.Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission cor-responding to beat frequency of 1THz.Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1THz beat signal.
PACS 42.79.Dj;
42.60.By;42.55.Px
1Introduction
Dual-wavelength lasers (or two-color lasers)have
been investigated by many researchers [1–6]because of vari-ous applications such as terahertz frequency generation [1–3],dual wavelength interferometry [7],THz-imaging [8]and wavelength multiplexing or switching.One way to achieve dual-wavelength laser operation is to combine a single laser with one or two external cavities.In these dual-wavelength lasers,one of the critical devices is the grating which acts as an external cavity mirror.Gratings that have successfully served this purpose include bulky gratings [1–3],distributed Bragg gratings [4,5]and fiber Bragg gratings [6].The dis-tributed Bragg gratings [4,5]were grown directly in the ver-tical cavity surface emitting lasers (VCSELs);therefore,the dual-wavelength lasers based on VCSELs with two mono-lithic gratings are more compact and stable than the dual-wavelength lasers with the other types of gratings.Espe-cially,for terahertz signal generation,the VCSEL-based dual-wavelength laser is advantageous because it can be operated in the two-color (or dual-wavelength)coherent regime [1],which is practically possible using a very short external cavity with high enough frequency selectivity.However,this kind of VCSEL-based dual-wavelength laser is expensive to develop because of its complex structure.
u Fax:+82-31-290-7204,E-mail:kslee@ece.skku.ac.kr
In this paper,we present a hybrid-integrated dual-wave-length laser using a sampled Bragg grating formed in a planar lightwave circuit (PLC).This new kind of hybrid-integrated dual-wavelength laser is a good candidate for the dual-wavelength source for terahertz beat signal generation be-cause it is cost-effective and has an external cavity,which is short and compact enough for the two-color coherent lasing.Since a good THz beat signal is necessary starting point for THz radiation generation,it is extremely important to demon-strate a stable hybrid-integrated dual-wavelength laser.We will experimentally demonstrate that this hybrid-integrated ECL [9–11]combined with a sampled Bragg grating written in a silica PLC exhibits strong dual-wavelength laser oscilla-tion with terahertz beat frequency.In addition,using computer simulation,we will also predict the stable terahertz beat signal generation.
2
Device fabrication
Figure 1shows the schematic of the proposed hy-brid integrated dual-wavelength ECL in which an FP-LD and a sampled Bragg grating are integrated on a silica PLC plat-form.The FP-LD was a spot-size converter integrated laser diode having low coupling loss.The front and rear facets of the FP-LD were coated with anti-reflection film (with reflec-tivity R ≤1%)and high reflection film (R =∼80%),respec-tively [11].The length of the FP-LD was 300µm .The center wavelength of the FP-LD was about 1290nm ,and the longi-tudinal mode spacing was 0.8nm .The sampled Bragg grating was written by exposing the PLC waveguide to a KrF excimer laser beam (λ=248nm )through a combination of a phase mask and an amplitude mask.The PLC waveguide was Ge-doped silica waveguide and hydrogenated at room tempera-ture and 100atm pressure for more than 4days to enhance photosensitivity.According to the diffusion theory,4days is enough time [9]to saturate the waveguide core with hydrogen molecules at 100atm pressure.
The PLC sampled grating should be designed such that two strong reflection peaks occur within the gain bandwidth of the FP-LD for the operation of the dual-wavelength laser.The separation of the two lasing wavelengths is determined by ∆λ=λ2B /2n eff Λs ,where λB is the Bragg wavelength,n eff is the effective refractive index of the silica PLC waveguide and Λs is the sampling period of the PLC sampled grating.
294Applied Physics B –Lasers and
Optics
FIGURE 1Configuration of a dual-wavelength external cavity laser with a sampled Bragg grating formed in a silica PLC waveguide
Thus,for n eff =1.45,the required sampling period Λs for gen-eration of 1THz beating signal is ∼103µm .The length of the sampled grating was adjusted to ∼10mm by using a shut-ter placed between the KrF excimer laser and the phase mask.One can also vary the strength of the grating by controlling the exposure time of the excimer laser beam because the strength is proportional to the UV exposure time.A number of PLC gratings with a pitch,Λ,of 450.91nm and different sampling periods Λs were written with the laser beam of fluence of 680mJ /cm 2at the repetition rate of 5Hz .
In an ideal case,when the gain experienced by the two frequencies fed back in the laser by the sampled grating is ap-proximately equal,and the two modes are perfectly mutually coherent,the output intensity I (t )is modulated as [1]
I (t )=I 1+I 2+2
I 1I 2cos (2πf b t ),(1)where I 1and I 2are the laser mode intensities for the two dif-ferent wavelengths,and f b is the beat frequency of the two wavelengths.In reality,the lasing modes are only partially mutually coherent,and the amplitude of the modulation term in (1)decreases accordingly.Thus,a good dual-wavelength design must exhibit high mutual coherence between the lasing modes in order to achieve strong THz radiation generation.
3
Experimental results
The PLC gratings were grown after 900s-irradia-tion.Figure 2shows the transmission spectra of the
PLC
FIGURE 2Transmission spectra of the PLC sampled gratings used in the dual-wavelength external cavity laser with two different sam-pling periods,(a )Λs =∼100µm and (b )Λs =∼150µm
sampled gratings used in the dual-wavelength external cavity lasers for two different sampling periods Λs =∼100µm and Λs =∼150µm .The two reflection peaks in Fig.2indicate that the PLC gratings act as an external cavity mirror feed-ing back two laser wavelengths.The two peak wavelengths of the PLC gratings with Λs =∼100µm are approximately at 1312.02nm and 1318.86nm ,as shown in Fig.2a.Those of the PLC gratings with Λs =∼150µm are 1313.08nm and 1317.02nm as shown in Fig.2b.The bandwidths of the reflec-tion peaks of the PLC gratings are measured to be ∼0.1nm ,which is limited to the resolution of the optical spectrum analyzer.The reflectivity of the reflection peaks ranges be-tween 80%and 94%.The wavelength spacing between the two peaks for Λs =∼100µm is ∼5.84nm ,while the spacing for Λs =∼150µm is ∼3.94nm .
We demonstrated two different dual-wavelength ECLs using the PLC sampled gratings.The optical spectra of the dual-wavelength external cavity lasers with the PLC sam-pled gratings corresponding to the Fig.2a and b are shown in Fig.3a and b,respectively.Note that the FP-LD,which runs multi-mode without the external cavity feedback,becomes stabilized at two laser modes with sidemode suppression ratio (SMSR)>32dB after the hybrid integration on the silica PLC platform.The oscillation spectra of the PLC-based dual-wavelength ECL were measured at 25◦and at an injection current of 60mA .Figure 3a shows that the dual-wavelength ECL (with the grating with Λs =∼100µm )emits laser out-puts with equal intensity (i.e.,I 1=I 2)at 1313.04nm and 1318.74nm .Figure 3b shows that the dual-wavelength ECL with Λs =∼150µm oscillates with unequal intensity (i.e.,I 1=I 2)at 1313.08nm and 1317.14nm .This indicates that the laser output of the first ECL (Λs =∼100µm )results in a beat signal with f b ∼1THz and that of the second ECL (Λs =∼150µm )yields to the beat signal with f b ∼0.7THz .However,the dual-wavelength laser with equal intensity is ideal because it gives the modulation depth of unity (see (1)).Therefore,the first dual-wavelength ECL shown in Fig.3a is better than the second dual-wavelength ECL for the terahertz beat signal generation.
The oscillation wavelengths of the dual-wavelength ECL were extremely stable and consistent with the Bragg wave-lengths of the PLC gratings.This is because the thermal sta-
LEE et al.Dual-wavelength external cavity laser for terahertz beat signal generation
295
FIGURE3Optical spectra of the dual-wavelength external cavity laser with different grating sampling periods for two different bias conditions,(a)Λs=∼100µm and(b)Λs=∼150µm
bility of the silica-based PLC gratings is excellent and the PLC gratings act as the external mirror of the PLC-based dual-wavelength ECLs.Also,the silica PLC waveguide was formed to be operating under the single-mode condition above 1300nm.Therefore,the two modes should be oscillating in the same fundamental spatial mode such that the mode overlapping is excellent.In this section,we experimentally demonstrated that our external cavity lasers,consisting of a FP-LD hybrid-integrated with a sampled Bragg grating writ-ten in a PLC,exhibit strong dual-wavelength emission whose corresponding beat frequency is about1THz.The beat fre-quency can be directly measured by an intensity autocorre-lator[2].But in this work we do not attempt to measure it directly;instead,we use computer simulation to demonstrate the terahertz beat signal generation from the hybrid-integrated dual-wavelength ECLs in the next section.
4Simulated results and discussion
Several two-color lasing regimes exists,among which the coherent two-color lasing regime is the most promising one for THz radiation generation[1].In order to operate the laser systems in this regime,it requires very short feedback loops and very narrow spectralfilters,preferentially integrated on a chip[1]as shown in Fig.1.To complement our experimental results,we use realistic computer simulations to assess the coherence properties and stability of our two-color source.We utilize the same simulation technology as in[1], with the core of the model based on a broad-band laser model capable of resolving semiconductor laser dynamics on very fast time scales while properly capturing all properties of the active structure[12].Our model consists essentially of two parts.Thefirst is the waveguide with the sampled grating,and the second is a Fabry–P´e rot semiconductor laser.Since we do not have access to details of the laser active structure,we re-sort to using an active structure model that should have similar properties,though the gain-maximum wavelength is slightly different from the lasing wavelength of the experimental laser. However,from the point of view of THz generation,details of the active structure are less important.In fact,the behav-ior observed here can be reproduced on a qualitative level with any similar active structure.On the other hand,the laser cav-ity and feedback properties are crucial[1].These parameters are,therefore,taken directly from the experiment.These pa-rameters are taken directly from the ly,we reproduce the mode spacing of the laser and both facet reflec-tivities.Also,we use experimental parameters for the sampled grating.We choose coupling coefficient that reproduces the experimental reflectivity spectrum by shifting the central fre-quency to match the gain peak of the model
laser.
FIGURE4Simulation results of the dual-wavelength ECL with a sampled grating withΛs=∼100µm.(a)Optical spectrum and(b)laser output power exhibiting deep modulation(f b=∼1THz)
296Applied Physics B–Lasers and Optics
Figure4a shows the simulated spectrum in the two-color operation regime.The two peaks correspond to two frequen-cies enforced by the sampled-grating feedback.The almost equal height of the two peaks means roughly equal output powers of the two lasing modes.Simulation shows that their relative intensitiesfluctuate only slightly on a nano-second time scale.To check the long-term stability,we have simu-lated the system over hundreds of microseconds.Indeed,the simulation showed that the two-color regime is stable and ex-hibits only smallfluctuations.
The mutual coherence of the two lasing modes can be seen as a beat signal in the output intensity shown in Fig.4b.The depth of the modulation is almost100%indicating both equal-ized power and high mutual coherence of the lasing modes. Long-time simulation showed that the mutual coherence is stable and exhibits only smallfluctuations.Among the various two-color lasing regimes classified by Matus et al.in[1],our regime can be characterized as a coherent two-color regime, with a small multi-mode contribution.Thus,our simulations strongly indicate that the proposed design is highly suitable for THz beat signal generation.
5Conclusion
We demonstrated experimentally and theoretically a hybrid-integrated dual-wavelength laser consisting of an FP-LD which is hybrid-integrated with a sampled Bragg grat-ing written in a silica PLC for terahertz beat signal gen-eration.The PLC sampled grating in this new design acts as an external mirror that forces the two laser wavelengths to share the same composite cavity,unlike the conventional dual-wavelength laser which often used two ing the PLC sampled gratings written with sampling periods of 100µm and150µm,we demonstrated two ECLs exhibit-ing simultaneous dual-wavelength emission corresponding to the beat signals of f b≈1THz and≈0.7THz,respectively. The dual-wavelength oscillation with a side-mode suppres-sion ratio>32dB has been demonstrated.Our simulation results also indicate that the two-color operation is stable and produces mutually highly coherent modes suitable for THz beat signal generation.Thus,our dual-wavelength ECL de-sign combines favorable manufacturing aspects with the de-sired high quality dual-wavelength operation.
ACKNOWLEDGEMENTS This work was supported by the Ko-rea Science and Engineering Foundation(KOSEF)grant funded by the Korea government(MOST)(No.R01-2005-000-10252-0)and by AFOSR MRI grant F49620-02-1-0380.JVM acknowledges support from the Alexander von Humboldt Foundation.
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