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For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.
©2011 Optical Society of America
1. Introduction
Terahertz (THz) radiation has been intensively investigated recently due to its potential applications in communications, biology and medical sciences, non-destructive evaluation, homeland security, food quality control, environmental monitoring, spectroscopy, and many more [1–3]. High performance stable THz-wave sources are therefore critical devices in THz systems for any related applications. Among many enabling technologies, photo-mixing or difference frequency generation (DFG) process presents the most cost-effective and convenient way to generate monochromatic THz-wave [4–8]. Photomixing and DFG are similar in system configurations yet their operation mechanisms are different. In a photomixing system, two continuous-wave (CW) laser beams with slightly different carrier frequencies interact through optical heterodyning in antenna coupled semiconductor photomixers (GaAs or InP) [9–11]. Photo-induced carriers affect THz-generation in photomixing. With the advent of frequency stabilized tunable semiconductor laser diodes, THz radiation below 1 THz is readily achievable with well-controlled spectral resolution for THz-frequency domain spectroscopy (THz-FDS) [9]. However the low damage threshold of photomixer/antenna limits the maximum THz power to nW~µW level. The complex phase lock circuits between two separate lasers add system cost and its tunable frequency range will not exceed 2 THz in most cases [11]. On the other hand, for DFG process, dual-wavelength near-infrared (IR) laser fields stimulate nonlinear polarizations in bulk nonlinear optical (NLO) crystals. Under suitable phase matching conditions, the second order nonlinearity induced parametric process leads to generation of coherent THz radiation and its frequency is determined by the difference of two near-IR lasers. An obvious advantage of nonlinear DFG scheme is that the generated THz power is scalable with high power pump lasers and NLO crystals with sufficient physical dimensions. The linewidth of generated monochromatic THz-wave could be well defined with suitable laser sources. Due to the inherent characteristics of ultra-wideband THz frequency tunability (from sub-THz to tens of THz) brought by NLO crystals, DFG-THz sources are very attractive for a wide range of applications [4]. It is anticipated that single frequency THz-wave with narrow linewidth and high spectral purity would fulfill demands of high resolution spectroscopy and imaging applications for optimized performances [12].
Till now, most pump sources of CW/monochromatic THz systems utilize two separate laser cavities or gain mediums to generate two different wavelengths. The system is thus complex in mechanical alignment and the spatial overlap of laser transverse modes is poor [9]. A dual-mode multi-section laser diode has been proposed for CW THz wave generation while it suffers from the compound-cavity mode competition and limited THz frequency tuning range [13]. It is well known that optical parametric oscillators (OPOs) are able to emit dual-wavelength radiation at arbitrary spectral range using flexible configurations [4–7]. Its high output power and wideband tunability are favorable for mid-IR or THz-wave DFG, nevertheless, the free-space optics based systems are bulky and less efficient.
With the extremely low loss property and excellent mode confinement capability, it is advantageous to utilize optical single mode fibers (SMFs) to construct compact single cavity fiber lasers to achieve simultaneous dual-wavelength lasing with improved stability and robustness. Intensive investigations have been conducted with fiber lasers for millimeter-wave generation and radio over fiber (ROF) communications [14–16]. However, because of the intricate cavity arrangement [15] or complicated cascaded fiber Bragg gratings (FBGs) [16], the reported wavelength spacing of dual-wavelength fiber laser is lack of tunability and its frequency difference is always constrained in the microwave/millimeter wave region. Therefore, in order to match the wideband capable NLO crystals, a single cavity dual-wavelength fiber laser with continuously tunable frequency difference from sub-THz to several THz is to be developed to fulfill the DFG-THz requirements.
We have demonstrated an ultra-wideband tunable THz-wave source based on DFG in organic 4-dimethylamino-N-methyl-4-stilbazaliumtosylate (DAST) crystals using dual-wavelength OPO pump source [4,17]. The refractive indices of DAST crystals grown in our lab have been measured to be almost identical from near-IR wavelength to THz-wave region [4]. The coherent length calculation according to the collinear phase matching condition reveals that a fiber laser emitting around 1.06 µm or 1.55 µm (corresponding to Ytterbium doped fiber or Erbium doped fiber, respectively) will be a good pumping source of DAST crystal for THz-wave generation. The exceptionally large second order nonlinearity (d11 = 230 pm/V, about ten times larger than conventional in-organic materials) makes DAST crystal extraordinarily promising for next generation high power and highly efficient THz downconversion. In this paper, for the purpose of fully exploiting advantages of fiber laser system and DAST crystal, we proposed and demonstrated a monolithic fiber laser pumped DFG-THz wave generation infrastructure. The single cavity dual-wavelength tunable fiber ring laser consists of chirped fiber Bragg gratings (CFBGs) based wavelength selector and a semiconductor optical amplifier (SOA) for laser oscillation. The local thermo-optical phase change along the linearly chirped FBG contributes to the seamless wavelength tuning. As a result, the CW dual-wavelength fiber laser emission around 1060 nm has been achieved with single longitudinal mode operation. With excellent beam quality and spatial coherence, the two-color fiber laser emission is then amplified by a polarization maintaining Ytterbium fiber amplifier (PM-YDFA) to pump the organic NLO DAST crystal. Collinear phase matched DFG in DAST crystal leads to the generation of CW THz radiations.
2. System configurations
The proposed fiber laser configuration and subsequent THz-wave generation scheme is depicted in Fig. 1 . Upper part (ring cavity) is the building block for dual-wavelength fiber ring laser, in which a fiber pig-tailed wideband 1060 nm SOA (Innolume Inc) is the gain medium. It features a high saturation output power of 18 dBm and a wide optical bandwidth of 60 nm (+ 15 dB gain). The optical isolator and polarization controllers (PCs) are used to maintain a unidirectional ring cavity and stable laser oscillation. The polarization state of laser propagating along the fiber is optimized for lasing operation and subsequent amplification. Two identical CFBGs are spliced into the cavity in which CFBG1 is used in transmission mode and CFBG2 is employed for reflection together with a optical circulator. The length of the CFBG is 2.5 cm and the linear chirp rate is 4.5 nm/cm. Therefore ~10 nm reflection/stop band is obtained (as illustrated in Fig. 2 ). Hydrogen-loaded photosensitive fiber (Nufern PS1060) for 1060 nm wavelength was used to inscribe the linearly chirped CFBG using the ultraviolet scanning beam technique. Based on thermo-optical phase shift in wideband CFBG, ultra-narrow bandpass optical filter and single frequency fiber laser near 1550 nm have been proposed [18–20] and we further extended this principle for dual-wavelength application around 1060 nm region according to the phase matching conditions of DAST-DFG process.
Fig. 1 Schematic system configuration. Upper part is the dual-wavelength fiber laser system and the lower part is DAST-THz generation system, respectively.
Fig. 2 Demonstration of reflection spectrum of CFBG2, transmission spectrum of CFBG1 and dual-wavelength transmission peaks of CFBG1 with heating wires. Offset is applied for clear illustration. Resolution: 0.01 nm.
As shown in the inset of Fig. 1, two NiCr resistance wires with cross-section diameter of 150 µm are collected parallel to a DC power supply to act as heating elements. The heating wires are placed perpendicular to the CFBG1 and touch the bare fiber at two different locations. In experiments, the fiber itself is fixed in the V-groove mounted on a copper heat sink to dissipate unwanted thermal influences. It is well known that the Silica glass fiber’s refractive index is temperature dependent and the tiny spot heated by the resistance wire will produce a relative phase change to the optical wave propagating through this fiber section considering negligible thermo-expansion coefficient during the heating process [21]. When the thermo-induced phase change is large enough to destruct the linear chirp relationship at that position of CFBG, the thermo-induced perturbation opens a very sharp transmission spike in the stopband of CFBG. Thus by mechanically controlling two heating elements simultaneously, two transmission peaks will appear. In order to reject the remaining transmission band out of the stopband of CFBG1, we employ an identical CFBG2 and a 3-port optical circulator to force only two thermo-induced transmission peaks circulating along the ring cavity. The fiber ring cavity has a length of 11.2 m and the longitudinal mode spacing of 17.86 MHz. Figure 2 illustrates the measured transmission spectrum of CFBG1, reflection spectrum of CFBG2 and an example of dual-wavelength transmission peaks in stopband of CFBG1 with heating elements applied, respectively. The transmission stopband and reflection spectrum of CFBG pair match perfectly and two ultra-narrow transmission spikes are well established (more than 20 dB sideband extinction ratio). Since the FBG has a linear chirp along the laser propagation direction, the shifting of heating points corresponds to a linearly changed wavelength transmission peak. The continuous wavelength tuning range is only limited by the transmission stopband of CFBG that can be expanded by improving FBG fabrication techniques. The full-width at half maximum (FWHM) of the thermo-optical filter is measured to be less than 10 pm, which is limited by the resolution of the optical spectrum analyzer. The exact spectral width of transmission peak of the thermal optical filter is determined by the FBG chirp rate, applied temperature, modulation depth of refractive index and heating profile around the heating point.
3. Experimental results: dual-wavelength fiber laser and THz-wave generation
When two heating resistance wires are moved along a linear traveling stage while keeping good contact with the CFBG1 (bare fiber), the central wavelengths of transmission peaks will shift according to heating positions. Once the SOA driving current exceeds a certain threshold (80 mA in our experiments), simultaneous dual-wavelength lasing occurs. Figure 3(a) shows the overlapped dual-wavelength fiber laser output from the 10/90 optical fiber coupler when the SOA driving current is 400 mA. One wavelength is fixed at 1058.6 nm and another wavelength is shifted continuously by changing the corresponding heating element position smoothly. The output power of each wavelength is identical and around 0 dBm. The wavelength separation can be adjusted from 0.3 nm to 9.5 nm, which corresponds to a frequency difference covering 80 GHz to 2.5 THz. Although the minimum lasing wavelength spacing could be less than 0.1 nm in principle by considering the FBG chirp rate and heating wire diameter, the thermal flow and phase error induces spectrum perturbation and the dual-wavelength fiber laser become unstable when we place the two heating elements even closer. Improving the heating profile and optimizing the thermal contact between the FBG and the heating element will be indispensable for future applications of our system.
Fig. 3 (a). Tunable dual-wavelength fiber laser output. (b): the relative intensity spectrum of two wavelengths.
In order to confirm the single frequency operation of laser output, we directly measured the relative intensity electrical spectrum of the two wavelengths separately by using a low noise photodetector with 1 GHz bandwidth and a 26.5 GHz RF spectrum analyzer (Agilent 4407B) from DC to 100 MHz span. Optical attenuator is used before photodetector to keep the detected laser power constant for two wavelengths. The resolution bandwidth (RBW) is 100 kHz. Since the SOA used in our fiber laser system was operated in saturation mode, its fast carrier recovery rate and gain saturation effect act as a high pass filter to suppress the low frequency beat noise and intensity noise [22]. The clean spectrum traces in Fig. 3(b) confirm single-longitudinal mode (SLM) operations with the help of ultra-sharp transmission window and the SOA mode suppression effect since no self-beating note can be observed. The relative intensity noise (RIN) is measured at −125 dB/Hz from 200 kHz to 1 MHz span.
The generated dual-wavelength laser is boosted to higher power by the following high power PM-YDFA (IPG Photonics: YAD-1-PM-SF). Since our fiber laser output is generally elliptical polarized, a polarization controller is used before the input of YDFA to achieve optimum amplification performance. Stable lasing and amplification performances can be maintained in lab environment. Figure 4(a) shows the optical spectrum of two-color fiber laser before and after YDFA when the frequency difference of two wavelengths is 1.5 THz. The driving current of YDFA is 1 A. More than 35 dB optical signal to noise ratio (OSNR) at 0.1 nm optical spectrum resolution is preserved after amplification. The relationship between the OSNR and the total output power from YDFA is also plotted in Fig. 4(b). The OSNR degradation is quite acceptable which is vital for an efficient DFG process.
Fig. 4 (a). Optical spectrum of dual-wavelength laser before and after YDFA (Resolution: 0.1 nm); (b): OSNR versus total pump power from YDFA.
Using this dual-wavelength fiber laser, a DAST-DFG system has been implemented to generate monochromatic THz wave radiation from sub-THz to 2 THz. As indicated in the lower part of Fig. 1, by adjusting a fiber-matched aspheric collimator, the linearly polarized amplified dual-wavelength fiber laser output is focused to 0.1 mm beam diameter on a DAST crystal with 440 µm thickness. Such a thin DAST crystal is selected to avoid large internal absorption of THz wave. The generated THz radiation is collimated by an off-axis parabolic mirror (focal length 50 mm) with a hole. The residual fiber laser emission after DAST crystal travels through the 1 mm diameter central hole and absorbed by a damper. Long wavelength pass filter and black polyethylene film are used to eliminate any remaining near-infrared pumps and stray light. The THz wave component is then focused by another off-axis parabolic mirror (focal length 254 mm) and detected by a 4K-Si bolometer. The a-axis of DAST crystal is set parallel to the incident pump polarization. The largest nonlinear-optic coefficient d11 is used to achieve type-0 collinear phase matching for DFG-THz process [4]. As an evidence of THz-wave emission, no THz-wave signal can be observed when the a-axis of DAST is rotated perpendicular to the pump polarization state. Lock-in amplifier (NF LI5640) is utilized to enhance the THz-wave detection sensitivity and 300 Hz chopping frequency and 10 ms time constant are set for measurement.
In Fig. 5(a) , THz output power versus incident fiber laser pumping power for 2 frequencies (0.6 and 1.54 THz) are demonstrated in logarithmic scale. The data traces can be well fitted by quadratic curves, a typical DFG feature. When the incident pump power is fixed at 0.85 Watt, THz power spectrum from 0.5 THz to 2 THz is obtained by shifting the heating elements while keeping the constant total pump power, as shown in Fig. 5(b). As a common feature of DAST-based THz generator, the lowest THz output is around 1.1 THz at which a very strong absorption peak of DAST crystal exists [4,7]. The maximum output power is obtained around 0.6 THz, which agrees well with previous investigations [23]. It suggests that, with fiber laser pumping source near 1 µm wavelength region, DAST crystal is effective to generate sub-THz wave (<1 THz) due to low attenuation and optimal phase matching conditions.
Fig. 5 (a). THz output versus pump power. Dashed lines are quadratic fitting curves; (b): THz output spectrum with fixed pump power: 0.85 Watt.
4. Conclusions
In conclusion, we proposed and successfully demonstrated a single cavity dual-wavelength fiber laser pumped DAST-DFG THz wave generation system. Using the wideband, inhomogeneous gain medium SOA, gain competition and laser beat noise can be effectively suppressed. The dual-wavelength lasing near 1060 nm with single longitudinal mode operation has been achieved and their frequency difference can be well controlled by the wideband CFBG based wavelength selector. Thermo-optical effect induced phase change along the CFBG leads to extremely sharp transmission window and two wavelength oscillations were established by mechanically adjusting two separate thermo-contact positions. As a result, continuous-wave THz radiation from 0.5 to 2 THz has been successfully generated using this newly developed dual-wavelength fiber laser source pumped DAST crystal. The continuous wideband tunability of this fiber laser is only limited by the CFBG spectrum and gain bandwidth. It has been demonstrated that highly efficient organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal and inorganic GaSe crystal are promising for efficient THz-wave generation, with pump wavelength around 800~1100 nm and 1530~1560 nm, respectively [24,25]. With the well-established FBG fabrication technique and suitable active fiber, our system can be easily tailored to match these NLO crystals. Combining the narrow linewidth and optical fiber-based infrastructure, our fiber laser based THz-wave system is expected to be promising for future compact, low-cost and high precision THz spectroscopic and imaging applications.
Acknowledgement
The authors would like to thank Y. Kamata, M. Saito, C. Takyu, Y. Konno and T. Shoji for their excellent technical supports.
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