A novel compact optical engine for a micro projector display is experimentally demonstrated, which is composed of RGB light sources, a tapered 3 × 1 Fiber Optic Color Synthesizer (FOCS) along with a fiberized ball-lens, and a two dimensional micro electromechanical scanning mirror. In the proposed optical engine, we successfully employed an all-fiber beam shaping technique combining optical fiber taper and fiberized ball lens that can render a narrow beam and enhance the resolution of the screened image in the far field. Optical performances of the proposed device assembly are investigated in terms of power loss, collimating strength of the collimator assembly, and color gamut of the output.
© 2010 OSA
Advances in projection displays and near-to-eye (NTE) displays have intensified interests of further miniaturizing micro projection displays that could be wearable. Micro electro-mechanical systems (MEMS) have been successfully adopted to provide high definition frames in projection displays. Digital micromirror device (DMD), as a family of MEMS, has been applied pervasively in digital light processor (DLP) based projectors . Alongside DLP, alternative projection technology of scanning micromirror projectors (SMP) has been also developed [2, 3]. The main difference between DLP and SMP is their color rendering technique. DLP micro display employs a sequential color system such as a color wheel, while SMP uses a color combiner and modulated light sources . Recently a commercial compact SMP has been introduced that can be embedded inside mobile communication devices . However, its RGB color combiner is still based on a dichroic prism (DP) which is bulky in volume imposing a significant limit on optical engine miniaturization.
Recently the authors’ group introduced novel application of fiber optics in the projection display devices [6, 7]. Fiber optic color synthesizer (FOCS) technology has shown very wide color gamut coverage and can be a feasible solution to miniaturize the RGB color combiners . As an alternative to optical fiber device, a waveguide type color synthesizer has been also demonstrated for a compact scanning micro projection display system . In the case of ridge type waveguide as in the reference , however, it is necessary to have additional launching optics to couple the light source into rectangular waveguides, which induces inherently large coupling loss. Ridge waveguide also suffers from waveguide loss due to surface roughness, which gets more severe in the visible range. FOCS is based on commercial hard polymer clad fiber (HPCF) that has a circular large modal area with a large full acceptance cone angle . Therefore, FOCS can efficiently couple both LEDs and lasers and guide them with a very low loss.
The resolution of an image in SMP can be enhanced by providing small beam spot size and divergence angle. By applying the optical fiber tapering technique  on the output fiber of FOCS the spot size and the divergence angle of the beam can be flexibly controlled to desirable values. Therefore, the flexibility and adaptability of optical fibers to manipulate the beam with a lower coupling loss of FOCS can provide a feasible novel route to miniaturization of projection displays.
In this study, we experimentally demonstrated a novel optical engine for a SMP, which consisted of a 3 × 1 FOCS for RGB light sources, a tapered fiber with a collimating fiber ball lens, and a 2-dimensional MEMS scanning mirror, for the first time to the best knowledge of the authors. In comparison to prior FOCS which was made by a star coupler [6, 7], we report a new branch type FOCS, with a special tapered lens tip integrated at the output. Projection characteristics for laser source and LEDs were experimentally compared using the proposed optical engine. The proposed device could provide a high potential for NTE displays, and head-worn displays (HWD) .
2. Design and configuration
2.1 System configuration
Figure 1 shows the schematic diagram of the proposed optical engine configuration. FOCS is made of a branch type 3×1 fused taper coupler using HPCFs. The polymer cladding was removed in the tapering process and individual silica core was tapered down to 122~126 μm near the fusion region. The fused zone has the length of 6mm and the silica output port diameter was 180 μm. The FOCS configuration was packed with a low refractive index polymer (PC-409). The HPCF used in this experiment provided by TEQSTM and it has silica glass core with a diameter of 200±5 μm surrounded by low refractive index polymer (PC-409) cladding of 25±5μm thickness. Overall diameter of Tefzel jacketed fiber is 275±5μm. The numerical aperture of HPCF is 0.39 and the core and cladding refractive indices are 1.457 and 1.407, respectively at the wavelength of 633 nm.
A 50μm pinhole in front of the taper tip blocks scattering noise from the FOCS. The collimated beam is projected onto the 2-dimensional MEMS mirror whose diameter is 1.2 mm and provided by Hiperscan (model: DM2Dk8). The MEMS mirror resonantly vibrates at certain mechanical frequencies in the horizontal and vertical directions to obtain scanning images at the screen.
2.2 All- fiber collimator design
The light guiding properties along the micron scale taper made of HPCF can be analyzed using the geometrical-ray tracing method [12,13]. The actual fiber taper profile is not linear and it is then simulated as a composite linear taper with two distinctive segments with different slopes as in Fig. 2 where ra, rb, and rc (ra > rb > rc) are the radii of the cross sections, Ω1 and Ω2 are the half-taper angles of the first and the second linear tapers respectively.
Simulation results of the ray tracing along the fabricated taper using LightTools® are summarized in Fig. 2. Here numerical data for the taper parameters were taken from the experimental measurements which are discussed in the next part. The acceptance angle of rays launched to the taper was found to be 7.62° at the wavelength of 550nm. The fabricated taper provided an excellent guidance of visible beams without leakage loss as shown in Fig. 2-(a). We have also included a hemispherical lens at the tip of the taper, along with 500μm ball lens in order to investigate the optical performances of the “all-fiber collimator” proposed in this study. The results are shown in Fig. 2-(b). Placing a 500μm collimating ball lens at the effective focal length of 403 μm from the lens tip provided a collimated beam with the diameter of 350μm for the incident beam at λ=550nm. Since the diameter of the hemispherical micro-lens is so smaller than the diameter of the ball lens (39μm/500μm ≈0.08), it can be approximated as a point source of light located in the focal point of the ball lens.
Figure 3(a) shows a simulation of the intensity distribution of the propagated beam at the position of the hemispherical lens using LightTools®. Figure 3(b) illustrates the distribution pattern after the ball lens, at the distance of 300μm from the center of the ball lens. The intensity patterns in Fig. 3 are the result of the LED light insertion at the central wavelength of λ=550nm. The LED in this simulation was coupled to the fiber input and was assumed as a 1 watt lambertian source with the illumination area of 800μm2 .
3. Device fabrication
3.1 Branch type fiber optic color synthesizer made of hard polymer clad fiber (HPCF)
To fabricate FOCS, three strands of HPCF were twisted together. Then they were fused and tapered by implementing flame brushing technique . In order to have a low insertion loss and high color mixing efficiency, a relatively large waist was formed in the fusion region.
By applying the total elongation length of 10 mm at the velocity of 8μm/s and total twisting angle of 360°, a uniform circular cross-section waist with the diameter of 180μm and the length of 15mm was obtained for a 3×3 fused coupler. The waist was further tapered down by using an electric arc-splicer (Ericson FSU 975) to form a conical output. Actual photograph of the fabricated device is shown in Fig. 4 . Note that the FOCS reported in this paper is the branch type, while the prior FOCSs were star coupler type [6,7]. Branch type has an advantage of easy fabrication and high tolerance against the fabrication process variations. The assembly of FOCS was immersed in a low refractive index PC-409 polymer and the taper part was covered by a thin layer of PC-409 (5~15 μm), then all cured by UV illumination.
3.2 All-fiber collimator using conical tapered lens fiber and fiberized ball lens
For efficient beam shaping, the output of FOCS was modified to form a conical taper whose length was 736 μm and its diameter varied smoothly from 176 to 39 μm. The magnified photograph of the tapered output of FOCS is shown in Fig. 5(a) . At the end of the taper, we formed a hemispherical lens with an approximate curvature radius of 20μm.
We also fabricated a fiber ball lens whose magnified image is shown in Fig. 5-(b). By utilizing high-current arc discharge of a fusion splicer, a spherical lens of ~500 μm diameter was formed at the cleaved end of bare HPCF. The hemispherical lens tip of the tapered FOCS output in Fig. 5-(a), and the fiberized ball lens in Fig. 5(b) constituted an all-fiber collimator to provide a collimated beam incident on the MEMS mirror as in Fig. 1.
4. Experiments and results
4.1 Light sources
We used two types of RGB sources, incoherent LEDs and coherent lasers. The peak wavelengths of RGB LEDs were 640, 524, and 463 nm. The illumination area of each LED was less than 800μm2 and their maximum luminous intensity was 4500 mcd. Butt-coupling method between LED and HPCF was adopted . 5 mW solid state RGB lasers at 635, 532, and 473 nm were employed and collimating-reducing pairs of convex lens were used to reduce the laser beam diameter to 420, 347, and 195 μm for red, green and blue lasers.
4.2 Light source-FOCS assembly
For the light source-FOCS assembly, we investigated its optical performances in terms of the transmitted power variation among ports and insertion loss. For both LED and laser sources, we injected one of RGB sources to each of three input ports of FOCS and measured the output power. The experimental results are summarized in Table 1 . In LEDs, the optical power in the port 2 showed the maximum throughput for the red and green, but the port 3 showed the maximum throughput for the blue. See Fig. 4. In lasers, port 1 showed the maximum throughput for red and blue, but port 2 showed the maximum for green.For both LED and laser inputs, the optical throughput power varied slightly from port to port. Despite highly multi-mode nature, the HPCF itself does have wavelength dependent transmission loss. Furthermore the fused-tapering process provides wavelength selective coupling constants for each of guided modes. These combined characteristics are attributed to the difference in the throughputs.
The insertion losses were also measured for LEDs, which were butt-coupled to cleaved ends of HPCFs, 34.08%, 41.92%, and 53.87% for red, green, and blue, respectively. The overall power loss of 56.66% was measured in the LED-FOCS assembly. To measure the insertion loss for the lasers, we maintained the collimating- reducing pair of lens. For the lasers, the average insertion loss of red, green and blue lasers were 6.89%, 10.87%, and 29.37%, respectively. The overall power loss of 21.99% was measured in the laser-FOCS assembly. The transmission characteristic of conventional HPCF shows that the transmission loss rapidly increases as the wavelength decreases from 600 nm to UV . As a result, FOCS made of HPCF showed an increment in the average insertion loss as the signal wavelength decreases from red to blue.
4.3 Collimator performance
Along with fiberized ball lens, we also tested a bulk ball lens made of BK7 glass with the diameter of 10 mm. The output of FOCS positioned at the effective focal point of the ball lenses, 403μm for the 500μm fiberized ball lens and 6.2 mm for the bulk ball lens. To quantitatively characterize the effect of fiber-taper on the collimator assembly’s performance, a measurable quantity, “collimating strength” (CS), was defined, which is the ratio of the divergence angle of the beam out of an untapered FOCS output to that of tapered FOCS output. By developing MATLAB program based on the second moment method of far filed beam size measurement  and using a CCD camera (Edmond eo1312) the spot size and the divergence angle of the beams in the collimator systems were analyzed.
Table 2 compares the two types of collimators for RGB laser sources. For the fiberized ball lens, an average spot size of 1.36mm was obtained at the distance of 50mm from the conically tapered fiber output. In the case of the bulk ball lens an average spot size of 1.23mm was obtained at the distance of 500mm from the output. It is noted that by tapering the fiber, we could significantly reduce the divergence angle by several folds, which is essential in projection display applications. Especially the bulk ball lens showed an average CS of 7.18 larger than 4.48 of the fiberized ball lens, which indicates that fiber taper-the bulk ball lens combination can provide a narrow divergence angle suitable for longer distance projection applications.For LED sources, the collimating performances are summarized in Fig. 6 , where the intensity distribution of the red LED beams are shown for two types of ball lenses combined with tapered and untapered fiber tips. In the case of fiberized ball lens, Fig. 6-(a), the intensity distributions were taken at the distance of 5 mm from the lens, and the bulk ball lens case, Fig. 6-(b), was at 50mm. Their intensity distribution comparison confirmed that the taper structure significantly reduced the beam size by several factors.
These collimating capability analyses confirmed the very important contribution of the fiber taper structure in the proposed system. It is also noted that there is trade-off between the working distance and the spot size reduction in the case of all fiber collimator composed of fiber taper and fiberized ball lens. Further optimization of ball lens diameter in all-fiber solution is being pursued by the authors.
4.4 MEMS mirror operation
The MEMS mirror used in this study was a two dimensional scanning mirror (model: DM2Dk8 from HiperscanTM). The circular mirror plate was made of a thin high reflective aluminum with thickness of 30μm and diameter of 1.2 mm. By using gimbals mounting technology, the suspended mirror can be manipulated by an alternative electrostatic force to deflect fast around two independent axes . The maximum mirror deflection angles were 5.8° and 14.5° for the fast and slow axes respectively. These deflection angles were achieved by driving the MEMS mirror at the resonance electrical frequencies of 18.365 and 2.615 kHz, for fast and slow axes, respectively. To drive MEMS two function generators (HP 33120A) rendered 1 VPP square waves with the duty cycle of 50%. A Two-channel fast high-voltage amplifier (Model F20AD, FLCE Electronics) then amplified the driving signals to 55 and 50 VPP for fast and slow axes frequencies. To operate the MEMS mirror at its resonance state a frequency sweeping technique was employed .
4.5 System performances
Integrating all the components, light sources, FOCS, collimator, MEMS mirror as shown in Fig. 1, we investigated the system performance of the proposed optical engine in two perspectives; color gamut, and lissajous patterns.
The color gamut of the proposed system for the LED and lasers on CIE 1931 diagram were measured by using a colorimeter (CS-100A, Minolta). The CIE 1931 diagram of the proposed device was compared with those which belonged to NTSC, a typical CRT, and a TFT-LCD based on fluorescent light bulb in Fig. 7 .
The diagram showed that the proposed device with LED, and lasers RGB sources, as described in Table 1, provided significantly wider color gamut than conventional displays, which is consistent with our previous reports [6, 7]. Note that FOCS remained the same for LED and laser color gamut measurements and therefore the color gamut could be further expanded with proper light sources. The maximum output power achieved by the LED-based display and laser-based display were 10μW and 2 mW, respectively. Hence, the proposed system could be used for near eye projection system by adopting LEDs and also used for far field projection system with lasers.
By activating the MEMS mirror, periodic fast projection of the beam imaged lissajous patterns on a screen. Figure 8 shows patterns for laser sources and bulk ball lens. The approximate projection area was 10×20 cm2 ~20×30 cm2 at the distance of 100cm from the system output. By changing the intensity level of primary colors and the oscillating frequencies ratio, various lissajous patterns with a wide gamut of colors were obtained, which shows high performance of color rendering of the proposed system. Figure 9 shows linear patterns with secondary colors of cyan, magenta, and yellow for the LED sources using fiberized ball lens. The length of projected lines on the screen was nearly 6cm at the distance of 20cm from the output. Further refinement of image for LED sources is being pursued by the authors.
A novel all-fiber optical component for micro-projection display was experimentally demonstrated which is composed of fiber optic color synthesizer (FOCS), fiber taper, fiberized ball lens. The overall power loss of 56.66% and 21.99% was measured in the LEDs and lasers in the FOCS assembly. By tapering output fiber of FOCS and concatenating it with a fiberized ball or bulk lens, we could control the projected beam spot size. For the 500μm fiberized ball lens, an average spot size of 1.36mm was obtained at the distance of 50mm from the conically tapered output. In the case of the 10mm bulk ball lens an average spot size of 1.23mm was obtained at the distance of 500mm from the output for laser sources. The maximum output power achieved by the LED-based display and laser-based display were 10μW and 2 mW, respectively. Various lissajous patterns were successfully achieved and efficient synthesis of color capability was also confirmed in a wide color gamut in CIE1931. A wide color gamut, low insertion loss, flexible beam control and ultra compactness of the proposed all-fiber optical engine can find ample applications in micro projection displays.
This work was supported in part by the KOSEF under Grant ROA-2008-000-20054-0 and Grant R15-2004-024-00000-0, in part by the KICOS under Grant 2009-8-1339 and Grant 2008-8-1893 (the European Community's Seventh Framework Program [FP7/2007-2013] under Grant agreement n° 219299, Gospel), in part by the ITEP under Grant 2008-8-1901 and Grant 2009-8-0809, and in part by the Brain Korea 21 Project of the KRF.
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