Experiments and Results - The Stable and Tunable Fiber Laser

Chapter 5 The Stable and Tunable Fiber Laser

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

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

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.

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

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

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.

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 2^nd 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.

Delavaux, “Stable single-mode erbium fiber-grating laser for digital communica-tion,” J. Lightwave Technol., vol. 11, 2021, (1993).

[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.

Commun., vol. 250, 163, (2005).

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

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

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

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

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)

( ) ( )

configuration. From Eq. (6.2), if a shiny region is within the field of view, the region presents large reflectivity and dominates both of the average intensity and the average amplitude. Consequently, with a possible shiny region it is necessary for traditional methods to pick up small average values of the intensity and the amplitude to avoid camera saturated. However, in general case the values are too small for some dull region to retrieve the phase from Eq. (6.3). By the point of view of mathematics, the intensity contrast could not affect the precision of the phase retrieved process; but the system is discrete and the small contrast will invoke large digitized noise during the phase retrieved process according to Eq. (6.3). This chapter presents a novel projection moiré system which could avoid contrast loss for inspecting the high variation range of surface reflectivity. The DLP chip not only adjusts the light intensity for the whole region but also adjusts the intensity pixel-by-pixel. The system

could optimize the average intensity and the average amplitude of each pixel. This function could ensure the whole region enough contrast gained for performing the phase retrieved process.

Fig. 6-1 The system configuration

In this study used Optoma EP728 DLP projector to create the signals of the fringe image with a resolution of 1024x768, and the fringe pitch was 3.3 mm (150 pixels). The camera that we used was SONY XCL5005 with a resolution of 2400x2014 and 12 bits/pixel. The frame grabber was DALSA X64 Xcelera-CL PX4 with a camera-link interface. The field of view in this system was 53x45 mm.

6.3 Results and Discussion

Fig. 6-2 The block diagram of a control unit

The block diagram of the intensity control unit is shown in Fig. 6-2. Firstly, Controlled the DLP to form uniform distribution of the light intensity and projected the light on the sample. Since there were shiny and dull regions on the surface of this sample, the image captured by the CCD ended up with dark in some areas and possibly with saturated in others. The calibration module received the raw image data from the CCD and an image processing algorithm indicated the boundary of the regions with different values of the surface reflectivity within the field of view and then resulted in the calibration factors for each region. The estimated factors were fed back to the intensity configuration module for the adjustment of the intensity of the fringe images and for the guarantee of the intensity of all regions being within the dynamic region of the CCD camera. For the industrial manufacturing process, the similar inspection condition would be assured that the boundary regions and calibration factors can be loaded from the database. For the phase-shifting algorithm, the modulated fringe images with the revision of the average intensity and the amplitude for different region from calibration factors were sent to DLP projector and projected onto the sample. Therefore several sets of raw images with the surface brightness levels of all pixels could be produced within the dynamic range of CCD camera. The raw images were reconstructed as high dynamic range images according to the calibration factors and were sent to phase-retrieving algorithm. Because of the images with larger signal-to-noise ratio from the high dynamic range, higher quality of 3-D data could be obtained.

To demonstrate the method, measured a slide mounted on a base plane with high reflectivity. Two grooves were formed with low reflectivity on the top of the slide by sand blasting. This study compared the measurement results between the traditional

fringe-projection moiré and the presented fringe-projection moiré system. The system included a DLP module, a uniform lighting module and a CCD camera. Firstly, aligned the system carefully and measured the relation between the pixel of the DLP and the pixel of CCD camera. Thereafter we could create a mapping table for looking up the intensity of the image which was captured by CCD camera according to the corresponding pixels on the DLP chip. The traditional fringe-projection moiré system and the DLP module provided uniform sine fringe over the whole field of the measurement. For solving the profile of the slide, the DLP module projected three sine-fringe images with a phase shift of 2π/3 to the slide; and then the three-step phase-shifting algorithm was used to solve the profile, as shown in Fig. 6-3. The cross-sectional plot of the marked region of the slide is shown in the left-hand side of Fig. 6-3, and the horizontal axis stands for the pixels of CCD camera and the vertical axis is the height of the slide. The left-bottom image is one of the fringe image captured by CCD camera, indicating that the low reflectivity on the top of the slide made the image lose its contrast, owing to the contrast of the fringe was insufficient to perform the phase-retrieving algorithm. There were many spark noises in the darkest region.

Fig. 6-3 The captured image and retrieved profile of the traditional fringe

77 projection

In order to overcome the insufficient contrast, the novel projection moiré system invoked the calibration module calculating the light intensity distribution over the field of view by modifying the edge detection algorithm and by defining the calibration factors for each region. Referring to the mapping table and using mixed-pixels algorithm, the measurement used the DLP module to adjust the average intensity and the average amplitude of the intensity modulation based on the factors and the fringes projected to the slide. The images captured by the CCD camera were then fed in the calibration module for the reconstruction of the high dynamic range

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