Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Sub-wavelength optical wires fabricated using CMOS (Complementary Metal–Oxide–Semiconductor) compatible techniques will lead the way to feasible and economic integration of optics and electronics. However, on the CMOS transistor layer, which necessitates thermal conduction through the silicon substrate, the SiO₂ layer is very thin, so having more than a few hundred nanometers of SiO₂ underneath the Si waveguide creates incompatibility. In addition, CMOS circuits are usually built on regular Si wafers instead of o called SOI (silicon on insulator) wafers. Here, we investigate a novel method to fabricate Si/SiO₂ waveguides from a regular silicon wafer rather than SOI wafer. It allows the archtect of great flexibility in design and fully compatible with the CMOS circuitry.

As shown in Fig. 1, deep silicon ridges were firstly created using typical lithography and dry-etching process. Then, high-power excimer laser was used to illuminate the Si ridges. Under proper illuminating condition, the Si ridges would be melted and reshaped to circular-profile structure. Finally, high-temperature oxidation process was used to oxidize the reformed structure. The final oxidation of the neck of this reformed structure prevents optical loss due to light coupling from the waveguide to the substrate. A structure with a uniform round cross section was formed. It exhibits a Si core with a diameter of 400 nm. The result shows the potential of integrating electronics and photonics on a single chip.

Fig. 1 SEM images of fabricating Si waveguides (a) a Si ridge from a regular Si wafer; (b) a circular structure after laser reformation; (c) the structure after first oxidation and removing the oxide; (d) the structure after second oxidation.

 Professor Sheng-Lung Huang's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The ultra-broadband nature of transition-metal-doped materials is eminently suitable for applications, such as petabit-bandwidth telecommunications and cellular-resolution optical coherence tomography. Various Cr⁴⁺:Y₃Al₅O₁₂ (Cr⁴⁺:YAG) lasers in bulk-crystal forms have been realized, yet poor thermal loading was identified as one of the key factors currently limiting the laser performance. This renders bulk forms difficult to implement as efficient, compact, and affordable laser systems. Recently, a double-clad architecture grown by the codrawing laser-heated pedestal growth (CDLHPG) technique was successfully developed for broadband light source and amplifier applications. The fiber based waveguide geometry features a high pump intensity and a better heat dissipation, resulting in a low-threshold Cr⁴⁺:YAG laser.

We report a significant advancement in the performance of a compact, efficient, low-threshold, and continuous-wave (CW) Cr⁴⁺:YAG double-clad crystal fiber (DCF) laser grown by the CDLHPG technique. To our knowledge the optical-to-optical slope efficiency of 33.9% is the highest ever reported for Cr⁴⁺:YAG laser at room temperature (RT) with just air cooling. This efficiency improvement was achieved by incorporating a strain-free Cr⁴⁺:YAG DCF. The strain-free feature also mitigates the photobleaching effect in Cr⁴⁺:YAG. Additionally, a self-selected linear polarized output (>100:1) was attained by simply adopting the orientated Cr⁴⁺:YAG gain medium without introducing any intracavity polarization-dependent components. The impact of crystal orientation on laser polarization and the slight pump-induced laser depolarization are also addressed.

Figure 1 shows the CW RT Cr⁴⁺:YAG DCF laser output power versus the incident pump power. The lasing threshold P_th was 78.2 mW with an optical-to-optical slope efficiency η_s up to 33.9%. The laser was operated at RT by just air cooling. The laser performance agrees well with the four-level Cr⁴⁺:YAG DCF model. By taking advantage of the strain-released crystalline core, the extracted best-fit values of emission cross section and fluorescence lifetime were improved significantly compared to that of strained core.

In conclusion, a Cr⁴⁺:YAG DCF laser with a 33.9% optical-to-optical slope efficiency and a 78.2-mW threshold is demonstrated. This achievement was obtained by reducing the distorted Cr⁴⁺ tetrahedron via strain relief and photobleaching mitigation. Realization of low-threshold lasing with high slope efficiency at RT for broadband gain medium at near infrared wavelength range represents a major step towards an affordable, compact, and efficient active device for applications ranging from telecommunications to biomedical applications.

Fig. 1. Cr4+:YAG DCF laser output power against the incident pump power achieved at RT. Upper inset: the corresponding lasing spectrum at the maximum incident pump power of 360 mW, showing a high side-mode suppression ratio of ~70 dB. Lower inset: stability of the Cr4+:YAG DCF laser measured every 100 ms without active cooling at RT.

Professor Chi-Kuang Sun

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Terahertz (THz) technology has high potential in a wide variety of applications ranging from high-speed telecommunications to bio-imaging. To facilitate THz applications, it is essential to develop low-loss and low-cost THz waveguides. Recently, we proposed a dielectric circular air-core pipe for low-loss THz waveguiding. The pipe waveguide is the simplest pipe with a large air core region surrounded by thin and low-index dielectric cladding. Without complex fabrication, we used commercial Teflon pipes to demonstrate that the circular pipe waveguides not only possess low attenuation constants (< 0.001 cm^-1) and high coupling efficiency (> 80%), but also suffer low bending loss. By modifying the circular pipe waveguides into rectangular ones, we further removed the transmission degeneracy of two orthogonal polarizations. It is expected that these easily available, low loss, low bending loss, and polarization-sensitive THz waveguides would have a high potential for THz sensing, communication, and imaging applications.

Fig. 1. Structure of the pipe waveguide. (n1 = 1, air).	Fig. 2. Attenuation spectra of straight Teflon pipe waveguides for t = 0.5 mm and 1.0 mm. The core diameters were the same (D = 9 mm).
Fig. 3. Bending loss spectra of the pipe waveguide for bending radius of curvature (R) = 75 and 60 cm.	Fig. 4. Attenuation spectra of PE rectangular pipe waveguides for different polarizations.

 Professor Gong-Ru Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A TO-46-can packaged VCSEL with a finite bandwidth of only 3 GHz has been employed to demonstrate a 10-Gbit/s all-optical NRZ-to-RZ data-format convertor, as shown in Fig. 1. This approach is based on the injection-locking induced gain-switching and the beyond-bandwidth electrical-pulse modulation of the TO-46-can packaged VCSEL biased at nearly lasing condition. To support 10-Gbit/s RZ data pulse generation, the resonant relaxation oscillation frequency of this TO-46-can packaged VCSEL is increased through direct modulation by an electrical-pulse at 10 GHz and external NRZ optical injection from 2 to at least 7.3 GHz. The optimized 10-Gbit/s RZ data-stream exhibits the signal to noise ratio, the timing jitter, and the pulsewidth of 7.2 dB, 2.9 ps, and 27 ps, respectively. The peak-to-peak chirp and corresponding chirp parameter are slightly raised to 4.3 GHz and 122 MHz/ps by increasing the injection power and the biased current. Owing to carrier accumulation and gain variation, the RZ data pulsewidth gets reduced by increasing the optical NRZ injection power and concurrently increasing the DC biased current.

A higher DC biased current further promotes the RZ data with greater modulation depth and sharper shape. A BER of 10^-9 at 10 Gbit/s is achieved at a receiving power of -19.5 dBm, and an improvement of receiving power penalties up to 16 dB is observed when the DC biased current is reduced to only 10% of its lasing threshold, as shown in Fig. 2. In particular, the slope of the BER versus receiving power significantly changes at a receiving power of >-19 dBm. The major reason for the slope change in the BER curves is crosstalk between the reflected incoming optical NRZ injection and the output of the VCSEL converted RZ data-stream. With a limited receiving sensitivity of the 10-Gbit/s lightwave receiver (Agilent 83434A), the VCSEL converted RZ data-stream inevitably introduces a noise floor in its BER response. Despite the finite bandwidth set by the homemade VCSEL, this work demonstrates the feasibility of operating the TO-can packaged VCSEL based RZ generator beyond its intrinsic modulation bandwidth with the aid of electrical-pulse modulation and externally optical injection. Most concerns regarding the noisy data-stream output can be addressed if the VCSEL can tolerate a larger bias and thereby achieve a better SNR before breakdown.

Fig. 1. VCSEL based 10-Gbit/s NRZ-to-RZ convertor. Amp: microwave amplifier; MZM: Mach-Zehnder modulator; OC: optical circulator; PC: polarization controller; TL: tunable laser; Att: attenuator; DSO: Digital Sampling Oscilloscope; BER: bit error rate tester; OSA: optical spectrum analyzer.	Fig. 2. BER analysis of the optical NRZ from a externally modulated VCSEL without injection (black), and the generated RZ at 10 Gbit/s from the injection-locked VCSEL at DC bias of 1.2, 1.3, and 1.4 mA.

Reference: 
Chia-Chi Lin, Yu-Chieh Chi, Hao-Chung Kuo, Peng-Chun Peng, Connie Chang-Hasnain, and Gong-Ru Lin, “Beyond-Bandwidth Electrical-Pulse Modulation of a TO-can Packaged VCSEL for 10 Gbit/s Injection-Locked NRZ-to-RZ Transmission,” Journal of Lightwave Technology, Vol. 29, Issue 6, pp. 830-841, Mar. 2011.