Chapter 4 The Supercontinuum Generation in a Tapered Fiber
4.4 Conclusions
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Supercontinuum generation from 1-µm tapered fiber using the 80 fs Ti:sapphire laser excitation is demonstrated experimentally and studied theoretically. By properly choosing the exciting wavelength, relatively wide spectra is observed from near UV to near IR only using 1-cm long and 1-µm-diameter optical tapered fiber. Besides, exciting power can be greatly lower down for wide spectra generation extended to near UV by properly connecting two fiber tapers. Split-step FFT method is investigated numerically in order to analyze the spectral response of supercontinuum generation phenomenon corresponding to the wavelength dependent loss occurred at transition region of the tapered fiber. The simulation results agree with the experimental results, and shows that the dispersion and nonlinear effects at transition region of the tapered fiber greatly influences the broaden spectrum shape. The theoretical result indicates that the zero dispersion cross point located at 2.6 µm so that the pulse width and peak power of the excited pulse is dramatically changed when propagates in transition region, which in term apparently affects the supercontinuum generation spectrum. Hopefully the simulation results in this work provide a helpful viewpoint to analyze the supercontinuum generation in typical tapered fibers.
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References
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Skryabin, M. Gnan, M. Sorrel, and R. M. De La Rue, “Soliton and spectral broadening in long silicon-on-insulator photonic wires,” Opt. Express, vol. 16, 3310, (2008).
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[11] T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation in tapered fibers,” Opt. Lett., vol. 25, 1415, (2000).
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“Coherence of subsequent supercontinuum pulses generated in tapered fibers in the femtosecond regime,” Opt. Express, vol. 15, 2732, (2007).
[15] R. Zhang, X. Zhang, D. Meiser, and H. Geissen, “Mode and group velocity dispersion evolution in the tapered region of a single-mode tapered fiber,” Opt.
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[18] M. Kolesik, E. M. Wright, and J. V. Monloney, “Numerical Simulations and Coherence Properties of Supercontinuum Generation in Photonic Crystal and Tapered Optical Fibers,” Appl. Phys. B, vol. 79, 293, (2004).
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“Analysis of thermo-optic tunable dispersion-engineered short-wavelength-pass tapered-fiber filters,” J. Lightwave Technol., vol. 27, 2208, (2009).
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Chapter 5
The Stable and Tunable Fiber Laser
5.1 Introduction
Single-longitudinal-mode (SLM) Erbium-doped fiber (EDF) ring lasers have potential applications in optical communications, fiber sensors, and spectroscopy. In accordance with these Erbium fiber lasers, the unidirectional ring-cavity structure, which can potentially offer more output power with low relative intensity noise, has been extensively studied [1] – [4]. Due to the requirements of intracavity components and connecting fibers, a rather long cavity length of the fiber ring laser is unavoidable and brings out an enormous number of densely spaced longitudinal modes lying beneath the Erbium gain curve. To complete single-longitudinal-mode operation, several single-longitudinal-mode fiber lasers techniques have proposed, such as using two cascaded Fabry-Perot filters into the ring cavity [5], employing a compound ring resonator composed of a dual-coupler fiber ring and a tunable bandpass filter (TBF) [6], and utilizing twisted EDFs and fiber-type half-wave plate to control the cavity [7], [8].
In this chapter, I propose and investigate experimentally a sable and tunable fiber double-ring laser to achieve single-longitudinal-mode operation, based on an Erbium-doped waveguide amplifier (EDWA), a fiber Fabry-Perot tunable filter (FFPTF), and a polarization controller into the ring cavity. Moreover, the output
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power, side-mode suppression ratio (SMSR), and the stabilities of power and wavelength of the laser are also discussed.
5.2 Experiments and Results
Fig. 5-1 illustrates the proposed single-longitudinal-mode Erbium-doped fiber double-ring laser. The proposed architecture consists of an EDWA, two 3 dB optical couplers (OCPs), a fiber Fabry-Perot tunable filter (FFB-TF), and a polarization controller (PC). The EDWA, which is manufactured via twostep ion-exchange process, has the advantage of inheriting the known properties of the Erbium-doped fiber amplifier (EDFA), such as low noise figure, slight polarization dependence, and no crosstalk between wavelength-divisionmultiplexing (WDM) channels. All optical performances are measured when the laser pump diode current equals to 440 mA at ambient temperature. The polarization controller is used to align the state of polarization of the ring cavity to guarantee a stable oscillation. The FFP-TF is an all-fiber device having a widely tunable range, low insertion loss of < 0.5 dB, and low polarization-dependent loss of ∼ 0.1 dB. This FFP-TF having the free spectral range (FSR) of 44 nm can provide wavelength selection in the ring laser cavity by controlling the external voltage (0 to 12 V) on the piezoelectric transducer (PZT) of this filter. In addition, an optical spectrum analyzer (OSA) with a 0.05 nm resolution is used to measure the output spectra of ring laser.
The FFP-TF not only determines a lasing wavelength but also serves as a mode-restricting component to provide the first restriction on the possible laser modes.
Because of the combination of a FFP-TF and a double-ring cavity, a single-longitudinal-mode operation in this fiber laser is achieved. The wavelength
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mode oscillates only at a single frequency, which satisfies the resonant conditions of the proposed structure.
Fig. 5-1 Proposed fiber double-ring laser architecture for SLM operation
The cavity of ring laser has a free spectral ranges (FSRs), FSR = c/nL, where c is the speed of light in vacuum, n is the average refractive index of the singlemode fiber of 1.468 and L is the total cavity length. The proposed ring laser has two ring cavities, as shown in Fig. 5-1. In this experiment, the two ring lengths of 11.96 and 13.04 m are the optimal choice for single-longitudinal-mode operation. Therefore, the lengths of two ring loops are 11.96 and 13.04 m long, corresponding to the FSRs of nearly 17.1 and 15.7 MHz, respectively. Then, the single-frequency operation of the fiber laser and its influence can be verified by a self-homodyne detection method. An optical circuit for a measurement is composed of a photodetector with a 3 dB bandwidth of 12 GHz and a Mach-Zehnder interferometer with a 25 km long standard single-mode fiber (SMF).
Fig. 5-2 illustrates the output wavelengths of the proposed fiber laser in an operating range of 1530 to 1560 nm. Fig. 5-2 also shows that all the output side-mode suppression ratios are above 64.6 dB. To realize the output behaviors of the laser, Fig.
5-3 shows the output power and side-mode suppression ratio versus different
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wavelength for the proposed laser over the wavelengths of 1530 to 1560 nm. Fig. 5-3 presents that the output power and side-mode suppression ratio of the laser are large than −5 dBm and 64.6 dB at 1550 nm in the effectively operating range. The maximum output power and side-mode suppression ratio (SMSR) of the laser are 4.3 dBm and 70.2 dB at 1536 nm, as also seen in Fig. 5-3. Compared with the past report [9], the proposed laser has the lower cost and simpler scheme. Moreover, the side-mode suppression ratio of the proposed laser is better (minimal SMSR of > 64.6 dB) than that of [9] (minimal SMSR of > 30 dB). Therefore, the proposed fiber laser not only has easily structure but also has better performance compared with the past.
Fig. 5-2 Output wavelengths of the proposed fiber laser in an operating range of 1530 to 1560 nm.
In order to investigate the performance of output power and wavelength, a short-term stability of the laser is measured in Fig. 5-4. An initial lasing wavelength is set at 1546.5 nm and total observing time is over 60 minutes. The results show that our proposed fiber laser has an excellent performance. The output power and central wavelength variations are less than 1 dB and 0.04 nm, respectively.
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Fig. 5-3 Output power and SMSR versus different wavelength for the proposed laser over the wavelengths of 1530 to 1560 nm
Observing Time (min.)
Fig. 5-4 Output wavelength and power variations of the proposed laser for a lasing wavelength of 1546.5 nm initially and an observing time of 60 minutes
To verify the single-frequency performance, the self-homodyne spectra of the fiber laser without and with double-ring structure (an operating wavelength is at 1546.5 nm) as shown in Fig. 5-5a and Fig. 5-5b, respectively. A noisy and unstable waveform with spikes is observed in the spectrum of single-ring laser as seen in Fig.
5-5a. When it is combined with a double-ring configuration, the proposed resonator can guarantee a single-longitudinal-mode laser oscillation in Fig. 5-5b.
Simultaneously, the fiber laser effectively suppresses sidemode frequencies of 500
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MHz, also shown in Fig. 5-5b.
Fig. 5-5 Self-homodyne spectra of the (a) single-ring and (b) double-ring laser at 1546.5 nm initially
5.3 Conclusion
This chapter has proposed and investigates experimentally a tunable and stable fiber laser with single-longitudinal-mode output based on double-ring architecture.
Double-ring structure provides a fine mode restriction and guarantees a
0 50 100 150 200 250 300 350 400 450 500
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single-longitudinal-mode operation. The output power of larger than −5 dBm and the side-mode suppression ratio of larger than 64.6 dB over the operating range from 1530 to 1560 nm can be obtained. And the maximum output power and side-mode suppression ratio of the laser are 4.3 dBm and 70.2 dB at 1536 nm. In addition, the power fluctuation of less than 1 dB and the central wavelength variation of less than 0.04 nm also are observed for lasing wavelength in a short-term observing time.
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References
[1] K. K. Chow, C. Shu, M. W. K. Mak, and H. K. Tsang, “Widely tunable wavelength converter using a double-ring fiber laser with a semiconductor optical amplifier,” IEEE Photon. Technol. Lett., vol. 14, 1445, (2002).
[2] R. M. Sova, K. Chang-Seok, J. U. Kang, and J. B. Khurgin, “Tunable dual-λ fiber ring laser based on 2nd order Sagnac-Lyot fiber filter,” IEEE CLEO 2002 Tech. Dig., USA 2002, vol. 1, 444, (2002).
[3] H. Ahmad, N. K. Saat, and S. W. Harun, “S-band erbium-doped fiber ring laser using a fiber Bragg grating,” Laser Phys. Lett., vol. 2, 369, (2005).
[4] F. Abdullah, A.S.M. Noor, M.A. Mahdi, H.A.A. Rashid, and M.K. Abdullah,
“Intracavity loss control effect on tuning range of tunable dual erbium-doped fiber laser,” Laser Phys. Lett., vol. 2, 535, (2005).
[5] K.J. Vahala, P. Namkyoo, J. Dawson, and S. Sanders, “Tunable, single-frequency, erbium fiber ring lasers,” Proc. IEEE LEOS 1993 Conf., USA 1993, 708, (1993).
[6] G.A. Ball, W.W. Morey, and W.H. Glenn, “Standing-wave monomode erbium fiber laser,” IEEE Photon. Technol. Lett., vol. 3, 613, (1991).
[7] V. Mizrahi, D.J. Digiovanni, R.M. Atkins, S.G. Grubb, Y.K. Park, and J.M.P.
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[8] J. Zhang, C.Y. Yue, G.W. Schinn, W.R.L. Clements, and J.W.Y. Lit, “Stable sinlge-mode compound-ring erbium-doped fiber laser,” J. Lightwave Technol., vol. 14, 104, (1996).
[9] H.C. Chien, C.H. Yeh, C.C. Lee, and S. Chi, “A tunable and single-frequency S-band erbium fiber laser with satyrable-absorber-based autotracking filter,” Opt.
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Chapter 6
Projection Moiré Profilometry with High-Dynamic Range Image
6.1 Introduction
Accurately measuring the 3-D shapes of the objects is important for the industry to speed up the product development and ensure the manufacturing quality. In general, the techniques of 3-D shape measurement can be classified into two categories:
contact-surface measurement and noncontact-surface measurement. The contact-surface techniques can provide the high accuracy for the measurement of any
“hard” objects which are insensitive to the optical properties of the surface. However, there are risks for the contact-surface techniques to damage the surface of the object inspected. Moreover, as a point-by-point measuring technique, the speed of the contact-surface technique is usually very slow. In contrast, the noncontact-surface methods would not damage the surface of the object inspected. Although among these two types of techniques different optical methods are extensively adopted, it is still an exceptional challenge for an object tested in an optical inspection system with a wide range of variation of the surface reflectivity. In addition, for increasing the speed of the measurement, the image detectors with low dynamic range are utilized typically providing 8 bits of brightness data only for each pixel. Hence the image captured by
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the imaging system ends up being too dark in some areas and possibly being saturated in others. Since the optical signal of the measuring region cannot be properly retrieved, these inspection methods would result in the loss of its accuracy.
An overview of 3-D shape measurement using various optical methods was provided by Chen [1]. The merits of the structured light method, also categorized as active triangulation, are (1) easy implementation, (2) fast full-field measurement, and (3) phase shifting with the fringe density and the direction change implemented without moving parts if a computer-controlled LCoS / DLP is used [2-4]. However, the optical properties of the object surface would affect the accuracy and thus a variety of optical 3-D shape measurement methods had been proposed for the shiny surfaces [5-6]. Nevertheless, for the object with very high dynamic range of its surface reflectivity, all these proposed methods might be potentially problematic.
Zhang and his coauthors addressed a high dynamic range (HDR) technique to measure this type of object [7]. They reported that multiple shots of the fringe images with different exposures were taken for each measurement. The final fringe images, used for phase retrieval, were produced pixel-by-pixel by choosing the brightest but unsaturated corresponding pixel form one shot. A phase-shifting algorithm was employed for computing the phase that can be further converted to 3D coordinates.
Therefore, the multiple shots taken can overcome the very high dynamic range of surface reflectivity; but it is oppositely a time-consuming measurement. On the other hand, Nayar suggested that using an optical mask adjacent to a conventional image detector array can achieve a high dynamic range image detector [8]. On the mask there was a pattern with the spatially varying transmittance, thereby giving adjacent pixels on the detector the different exposures to the scene. The captured image was mapped to a high dynamic range image by using an efficient image reconstruction
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algorithm; however, this method must downgrade spatially resolution for gaining a high dynamic range image.
In comparison with the previous studies, this chapter presents a technique for a low dynamic range imaging device, such as a CCD camera, to acquire a high dynamic range image in one-shot. Thereby, it is possible to measure a very wide range of the surface reflectivity without any reduction in the spatial resolution. The availability of the extra bits of the data at each image pixel enhances the robustness of the phase-retrieving algorithms so that an accurate surface topography of a measured object can be obtained. A digital-light-processing (DLP) is used as the light modulation for the control of the distribution of the light intensity when a sample is in higher reflectivity regions but under lower light illumination. The dull regions are illuminated with higher light intensity to produce a raw image whose surface brightness levels for all pixels are ranged within the dynamic range of a CCD camera.
Thereafter, the single raw image is processed by a compensation operation according to an intensity gain ratio of the light intensity before and after being modulated by DLP. As a result, a high dynamic range image can be obtained from the low dynamic range imaging CCD. Since this system only requires its imaging device to capture one image for processing, the advantages are not only low time-consuming and low errors during multiple sampling but also high spatial resolutions. This proposed technique is not limited to 3-D shape measurement systems; it is applicable to any optical measurement techniques with variant spatial brightness.
6.2 Measurement Method
This work developed a projection moiré system for inspecting the high variation
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range of surface reflectivity with the high speed measurement and preserving the spatial resolution. This system is based on a digital fringe projection and is associated with three-step phase-shifting algorithm. It retrieves the phase value of the fringe images and converts to 3-D shape. The basic configuration of the moiré system using digital fringe projection is shown in Fig. 6-1. A lamp is used for providing a uniform intensity distribution onto the DLP chip and then the modulated light is projected onto the object through a telecentric lens. The DLP chip controlled by a computer generates the fringe images which are projected onto the object under measuring.
These fringe images are distorted and reflected by the object and then captured by a CCD camera. The DLP chip not only generates the fringe images for phase-shifting method but also adjusts the light intensity distribution to be ranged within the dynamic range of the CCD camera. Then a frame grabber, installed in the computer, acquires the digital fringe images through a camera-link interface. The computer processes the fringe images obtained to retrieve the phase by using both of the phase-shifting algorithm and the phase-unwrapping algorithm with further conversion to 3-D coordinates [9]. According to the three-step phase-shifting algorithm, the original projected fringe image intensities are presented as
( ) ( ) ( ) ( )
where α is the DC component or average intensity, β is the amplitude of the intensity modulation, and φ is the phase of the spatial modulation. The fringe image is reflected by the surface of the object under test and then is captured by the camera. Those fringe images actually captured by the camera are
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where r(x,y) is related to the reflectivity of the object and the camera sensitivity. (x,y) is the phase of the fringe images after modulated by the object. The phase can be retrieved through Eq. (6.2) from Eq. (6.3)