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how to generate a uniform light distribution in a small solid angle

We want to using some kind of SLM(spatial light modulator) to generate an uniform optical intensity distribution (in a small solid state angle, about 0.2 rad).I am not sure that can be realized by one beam incident on the  DMD and control the reflect angle of each micro-mirror. The area of the expected intensity distribution are not so large (1 or several mm square or should be enough for our application. We using some laser light near 1060nm (average power is about 2 watts).

 Hopefully, you have some experience share with us about our application. We hope to make the reflective light can be incident on the facet of one multi-mode fibre with equal intensity in the each small direction or small solid angles, this is our motivation.

  • Hello Juqiang,

    First, welcome to the DLP section of the TI-E2E community.

    Could you answer some of questions to clarify:

    1. To clarify, you plan to control the light in a 0.2 radian diameter cone (~ 11.5 degrees).  Is this correct? 
    2. Do you plan to re-image the surface of the DMD on the fiber or are you simply wanting to have a cone of light?
    3. Which DMD were you thinking of using?

    DMD's only have two operational states, +12° and -12° of tilt for the µ-mirrors.  The cross-over transition from one side to the other takes about 1.5 - 2 µs and about 10-13 µs of mechanical settling time.  The mirrors cannot be controlled in an analog manner, only a binary manner.

    It sounds like you are contemplating mazimizing the throughput of a mutli-mode fiber by controlling the input.  In this case you would likely want to use optics to re-image the DMD surface at the fiber entrance.

    Fizix

  • Dear Fizix,

    Thank you so much for your useful reply.

    As for your questions, answers are follows:

    1.Yes, I want control the light in a radian diameter cone( 11.5 degree is rough value, at first we need the DMD can be work in principle as the way we expected ).

    2.Initially, We just want to have a cone of light.

    3.I am lack of experience for the selection of the DMD, could you please give us some suggestion ?

    According to your description, a binary manner should also be useful, when the mirror's tilt angle changed from 12 to -12 degree, it should be also scanning over the whole range (from 12 to -12 degree), if most of the process is finished with some kind of constant speed, it will be make sense. We can using some fast photodiode  and oscilloscope for detecting.

    In my memory, DMD can be used in laser display or laser TV or laser projector. How it controlled to scanning the micro-mirror and reflect the laser light to different area of the large screen ? so our application is some kind of similar with this one.

    BTW, maximizing the throughput is not the most important aim for us, but the uniformity should be the right one. 

    Yours sincerely,

    Fuqiang

  • Hello again Fuqiang,

    The cone of light without re-immaging the DMD on the fiber will be very challenging because using monochromatic light on the periodic structure of the DMD array will result in discreet orders in angle space.  These orders can easily be re-imaged at an image plane in the same way incoherent light would.

    However, t in order to create an arbitrary pattern of "pixels" in angle space would require controlling both phase and intensity and putting the Forrier transform of the desired pattern on the DMD.  This is a not a trivial task.

    It is true that when the mirror moves from on state to the other it passes through 24° (-12° --> +12°  or  +12° --> -12°) this takes only a couple of µs but then the µ-mirrors must mechanically settle for the next 11 - 13 µs.  During this time it is bouncing slightly (± several degrees) on the springtips from the final landed position.

    In DLP based systems, grayscale density is created by using Pulse Width Modulation [PWM] within the integration period of the sensor (human eye or camera).

    I hope this is helpful,

    Fizix