埋入菱形溝槽內之光纖透鏡模組與直立式相位型微光柵

Abstract

因應信息處理的蓬勃發展，快速的資訊存取時間、小型化的伺服系統以及高儲存密度成為光儲存科技發展的指標。為了縮減光學讀寫頭的尺寸與重量，目前已有多種微型光學系統已被提出，然而幾乎都有著光效率不佳的問題。既然光資訊儲存密度取決於雷射光源的波長，以藍光雷射（405nm波段）為光源的微型光學系統將逐漸被應用於光學讀寫頭上。

在現有的微型光學讀寫頭系統中，雷射光源通常建構於系統內，但是由於雷射光源所散發的廢熱甚少佚失至外界，易於發生廢熱積聚的現象，使得雷射光源的壽命因而減低，此外光學系統的光軸高度幾乎完全取決於雷射二極體活動層的高度，因此提出埋入菱形溝槽內的光纖透鏡模組，利用光纖透鏡模組將光束有效地導入讀寫頭系統中，如此雷射二極體便可從系統中分離出來，廢熱也因此能有效地被佚失至系統之外，加上菱形溝槽適於光纖精確定位與光軸高度的控制，廢熱積聚與光軸調制的問題便可獲得解決，並且可藉由光纖透鏡提供近似圓形的光束及縮小邊射型雷射光源的發散角，而達到光束整形的功效。

在現有的自由空間型微型光學讀寫頭中，微型光柵的成分為多晶矽，但是藍光對多晶矽的穿透性不高，因此系統的光學效率極低，於是我們設計一個在藍光波段具有高穿透效率的三維微光柵以作為光學讀取頭的關鍵元件，其光柵部份由對藍光波段具高穿透性的Si3N4構成，並由光學軟體G-SOLVER 估算出光柵厚度約為0.75um可在較大製程公差的情況下取得合適的繞射效率分佈（0階與+1或-1階繞射光束的光效率比值約為6∼12）。根據前述的設計，將此三維微型光柵利用微光機電製程來實現，並從此微型光柵之繞射光束的量測結果得到光效率比值約為5，藉由提升微型矩形光柵的製程品質來減輕梯形化現象，則此光效率分佈的誤差即可獲得改善。

With the rapid progress of information processing, fast access time, small-size drives and high recording density are main driving factors of optical data-storage techniques. To reduce the size and weight of the optical pickup-head, several kinds of micro-optical systems have been proposed, but the issue of low light efficiency exists in those systems. Since the storage density is determined by the wavelength of laser, the blue-laser (405 nm) will be widely adopted as the light source of the micro optical pickup-head system.

At present, the laser sources are placed in the pickup-head system. Since the heat dissipation rate of the laser diode is quite slow, thermal accumulation decreases the lifetime of laser diode. Besides, the optical axis height is constrained by the active layer of the laser diode. To solve these issues, the module with a fiberlens inserted in the rhombus-shaped channel is proposed. With this module, the beam is guided into the pickup-head, so the laser diode can be separated from this system. Since the laser diode is outside the pickup-head, the heat can be dissipated efficiently. Besides, the rhombus-shaped channel is suitable for the fiber-alignment and the control of optical axis height. Therefore, the issues of thermal accumulation and limited optical axis height are improved. Additionally, as a beam-shaping component, the fiberlens yields a more circularly-shaped incident beam with small divergence angles.

In the free-space micro pickup-head system proposed by M. C. Wu, the micro-grating is made of the poly-silicon. However, the poly-silicon is nearly opaque in the blue-laser spectrum, so the light-efficiency is quite low. Therefore, we designed a novel 3-dimensional micro-grating as one of key components in the pickup-head. The ridges of microgratings are made of the Si3N4, since the blue-laser is transmissible through the Si3N4 layer. Diffraction efficiency distribution of microgratings is suitable for the pickup-head (the diffraction efficiency ratio is from 6 to 12 between 0 and +1 or -1 orders), when grating thickness is 0.75um according to the simulated results of G-SOLVER software. According to this design, the 3-dimensional micrograting was realized by the MOEMS process. Finally, measurement results of the three-beamed microgratings showed that the diffraction efficiency ratio is 5, while the deviation of diffraction efficiency distribution might be reduced by trimming the profile of microgratings from trapezoid to rectangular shape.