A.         Next generation optical communication technologies

Research highlights of our most important achievements on optical communication researches can be summarized below with the reference to the representative publication list given in the beginning of Appendix II. Our full list of publications can also be found in the later part of Appendix II.

[Research Highlights]

I. Novel Optical Transmission & Processing :

(a)    Bi-directional fiber transmission and novel interleaver applications [1-5]

(b)   Novel all-optical signal processing (slow lights, Optical 2R, PLC ROADM ….) [6-12]

(c)    Cost-effective/high-performance duobinary/DPSK/…. Transmission [13-19]

II. Novel Optical Networking Architectures & Technologies :

(a)    Hybrid wireless/fiber access networking [20-24]

(b)   Optical access networking (PON, OCDMA, …) [25-28]

III. Novel Fiber Devices and Laser Sources :

(a)    Complicated fiber Bragg grating devices: advanced design and fabrication. [29-32]

(b)   Novel tapered fiber devices, fiber amplifiers and fiber lasers. [33-40]

(c)    High repetition rate modelocked fiber soliton lasers [41-43]

IV. Novel Theories & Applications :

(a)    Quantum squeezing and quantum entanglement of optical solitons [44-47]

(The reference numbers above and below are refered to the representative publication list given in the beginning of Appendix II.)

Some explanations of these achievements are given below:

I. Novel Optical Transmission & Processing :

In terms of research infrastructure, we have established the best fiber-circulating-loop testbed in Taiwan and have performed many researches based on this platform [see the lab photos below]. One good example is our study on the bidirectional fiber transmission [1-3]. After 500km transmission, the receiving sensitivity penalty is only 1.5 dB, comparable to conventional single direction transmission performance. In this study a novel 4-port DWDM wavelength interleaver is used to reroute bidirectional transmission into unidirectional amplification, so that the backscattering noises in the fiber amplifiers are blocked and high optical signal-to-noise-ratio (low cross-talk) is achieved.

Other good examples are the studies on the DWDM interleaver applications [4-5] and on the Planar-Lightwave-Circuit (PLC) Reconfigurable-Optical-Add-Drop Multiplexer (ROADM) [12]. We have studied the cascading transmission performance of a PLC reconfigurable OADM module in a fiber circulating loop. After 1100 km transmission, the power penalty for all channels < 2.25 dB and less than 2 dB sensitivity variations in cascaded transmission traffic is observed. The accumulated chromatic dispersion becomes obvious when the wavelength is detuned ±11 GHz. This technology can accommodate 32 channels simultaneously. The work was cooperated with Prof. Y.J. (Ray) Chen of UMBC in USA and with ITRI in Taiwan. This is a good example of our external research cooperation.

For optical transmission, we have developed several efficient schemes for generating different modulation formats (duobinary, DPSK, DQPSK, baseband digital/radio, etc) by using only a single EO modulator. [16,18-19]  These schemes can provide economic solutions for utilizing these modulation formats in practical applications. We have also obtained good theoretical results on the convergence of phase noises in DPSK transmission systems by using novel phase noise averagers [15,17]. This new technique is expected to greatly improve the DPSK receiver performance.  

For optical signal processing, we have studied an EDFA-free all-optical 2R regeneration scheme based on a compact self-seeded Fabry-Pérot laser diode (SSFP-LD) [7-9] [See the following figure for the setup]. The proposed 2R regenerator achieves a straight line transmission at 10 Gb/s over 76 km without either the EDFA or the external probe laser, both of which are traditionally required. The proposed compact 2R device has data-rate transparency up to 10 Gb/s and wavelength preserving operation (without wavelength conversion). In addition, we observed eye diagrams of the signal: (a) 2R-regenerated at 38 km; (b) 1R-regenerated at 38 km; (c) after 76 km propagation with 2R regeneration; and (d) after 76 km propagation with 1R regeneration. By using the proposed method, the power penalties, compared with the back-to-back case, were 0.65 and 0.9 dB after transmission over 38 km and 76 km, respectively, at BER = 10^-9. However, the 1R-only transmission has larger power penalties of 1.5 and 3.4 dB after transmitting over 38 km and 76 km, respectively, at BER = 10^-9.

Recently we have also made important breakthrough on the tunable QD VCSEL slow light devices. The achieved bit-rate*delay-time product was the highest among the similar semiconductor devices. Although still not sufficient for optical buffer applications, they may find use in some signal processing applications.

II. Novel Optical Networking Architectures & Technologies :

Examples of generated QAM OFDM signals at 15 or 20GHz and their performance are illustrated in the following three figures.

We have also further developed a frequency quadrupling technique illustrated in the following three figures and have successfully demonstrated the generation of 60 an 72 GHz microwave signals.

The above techniques have been employed to develop new hybrid wireless/fiber access networking system [23-24]. One of the main advantages is that no narrowband optical filtering is required.

        We have also developed several new techniques for other types of optical access networks. They include the Passive Optical Networks (PON) and the Optical Code Division Multiple Access Networks (OCDMA) [25-28]. The following figure illustrates the developed new 2D OCDMA light sources by external injection of a semiconductor Fabry-Perot laser.

III. Novel Fiber Devices and Laser Sources :

We have developed several new types of fiber devices, fiber amplifiers, and fiber lasers [33-37] One example is a novel tunable Er-doped fiber amplifiers covering S and C + L bands over 1490-1610 nm based on discrete fundamental-mode cutoff filters. [36] We demonstrate thermo-optically tunable Er3+-doped fiber amplifiers covering S- and C + L-bands (1490 ~ 1610 nm) using fundamental-mode cutoff filters discretely located in a 17.5-m-long standard Er3+-doped fiber. The maximum signal gains are measured to be 18.92 dB, 37.18 dB, and 15.19 dB with 980 nm pump power of 135 mW in S-, C-, and L-bands, respectively. The principle of the fiber filters is based on the fundamental mode cutoff mechanism illustrated in the following figure:

The achieved performance of the S-band amplifiers and lasers are illustrated below. We demonstrate a widely tunable fiber ring laser over 1451.9 ~ 1548.1 nm with tuning efficiency as high as 57.3 nm/°C using a 16-m-long standard silica-based erbium-doped fiber under 980-nm pump power of 208 mW. In principle, such a technique can be applied to other fiber laser systems to achieve shorter amplifying/lasing wavelengths that can not be achieved by conventional methods. In particular, the achieved lasing wavelength can be as short as 1451.9nm, which should be the shortest lasing wavelength of Er-fiber lasers reported to-date. Such a wavelength range is of particular interest to biomedical applications. This is why the paper is selected by the Virtual Journal of Biomedical Optics.[33]

At NCTU we have established the best advanced fiber Bragg grating (FBG) fabrication platform in Taiwan and have developed several advanced FBG design and fabrication techniques.[29-32]

We have also developed several new types of high-repetition-rate modelocked fiber soliton lasers and studied their laser dynamics.[38-40] In particular, we have observed new bound soliton phenomena in a high-repetition rate modelocked fiber soliton laser. The time separation of the bound solitons can be modulated by adjusting the RF driving power. This property should be useful for implementing new applications with this new type of bound pulse sources.

IV. Novel Theories & Applications :

 We have pioneered the development of the quantum theory for soliton squeezing, correlation, and entanglement.[41-44] We show for the first time that the solitons after nonlinear interaction are indeed quantum mechanically entangled. This should open new ways for generating quantum entangled light states.

B.         Next generation optical storage technologies

We have studied the power throughput enhancement effects of nano-aperture[45, 50]. We presented a ridged aperture encircled by a groove to allow the hybrid effect of coupling surface plasmon resonance to a propagating wave. This great improvement was demonstrated by its higher power throughput of 0.32 in the far field, a factor of 1.88 to the single ridged aperture that provided a signal-to-noise ratio of 20 dB in the near field.

We demonstrated a novel fiber-based near-field optical head consisting of a straw-shaped writing probe and a flat gap sensing probe [46]. The strawshaped probe with a C-aperture on the end face exhibits enhanced transmission by a factor of 3 orders of magnitude over a conventional fiber probe due to a hybrid effect that excites both propagation modes and surface plasmon waves. In the gap sensing probe, the spacing between the probe and the media surface functions as an external cavity. The high sensitivity of the output power to the change in the gap width is used as a feedback control signal.[48] We characterize and design the straw-shaped writing probe and the flat gap sensing probe. The dual-probe system is installed on a conventional biaxial actuator to demonstrate the capability of flying over a disk surface with nanometer position precision. [46]

For the self-alignment process, two types of nano-aperture, circular and C shapes, are fabricated by Focused Ion Beam (FIB) to combine with SIL/SSIL formed by thermal reflow.  Also, several novel micro actuators made of metal and polymer are developed, including a fabrication platform for three-dimensional polymer microstructures. Furthermore, based on extrusion and surface tension modulation techniques, a novel fabrication method to fabricate SILs on suspended cantilever array is successfully demonstrated. From the measurements of the power throughput and spot size, the proposed self-alignment process is successfully verified. For example, the φ15μm SIL/φ329nm circular aperture component is calibrated and found to enhance throughput 168%, comparing with that of φ329nm circular aperture alone. Furthermore, the throughput of 303nm×205nm C-shaped aperture/φ15μm SIL component can be enhanced 2443.8%, comparing with that of φ148nm circular aperture alone. These results verify the feasibility of the proposed self-aligned process [51]. 

For Near-field flyability, computational results show that grooved disks generate smaller pressure than smooth disks since grooves can accommodate air molecules and tune air pressure.[62] Further, flying higher makes pressure magnitudes closer between grooved disks and smooth disks in negative pressure area on slider bottoms. The proposed computational method facilitates pickup head design and improves head flyability during data read/write [56-61]. Experiments are carried out to demonstrate that the proposed controller performs better in flying height control than an optimal sliding mode controller. Accordingly, the controller can achieve stable flying height control in the presence of certain frequency vibration of optical disks. The media in near-field optical disk drives is usually made of glass or polycarbonate, which may generate significant deformation arisen from disk rotation. Using the advanced control method developed in this study can maintain stable and constant flying height, which is required in near-field data reading/writing. An optical lever method using only photo detectors for measuring flying height is applied to the flying head experiments. The results are validated by comparing with those from laser Doppler interferometers (LDV).