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ONET4211LD: About optical output (mW) power control of ONET4211LD

yoshihito takahashi
Prodigy 180 points
Part Number: ONET4211LD

About optical output (mW) power control of ONET4211LD Please tell me in detail.

 

The pins related to optical output (mW) power control are recognized as follows.

(1) pin 11: MODSET

(2) pin 12: MODTC

(3) pin 21: IBMAX

(4) pin 23: APCSET

* For other pin 19 PD, pin 20 CAPC, etc., follow the instructions in the data sheet. Wiring is done.

The registers related to optical output (mW) power control are recognized as follows.

(1) R MODSET

(2) R MODTC

(3) R BIAS MAX

(4) R APCSET

Even if I check the data sheet, I didn't really understand how to determine the values ​​of registers (1) to (4).

The specifications of the laser diode used are as follows.

Wavelength 　λ [nm]: 637 (typ.)

Average Optical Output Power 　PAVG [mW]: 0.5 (my request)

Threshold current 　ITH [mA]: 15 (typ.)

Laser-to-monitor transfer 　ρMON [mA / mW]: 0.05

Laser slope efficiency 　η [mW / mA]: 0.94

I want to set the light output of the laser diode to 0.5mW. In that case, If should be set to 11mA.

* Operate below the Threshold current to suppress the optical output.

Bias current is 6mA, Modulation current 10mAp-p If you set it to, I think you can set If to 11mA.

(1) R MODSET

To set the modulation current to 10mAp-p

From R MODSET [Ω] = 265V / I MOD [A] × (1 + TC × (T [℃] --T0 [℃]))

= 265V / 10mA × (1 + 4000ppm / ℃ × (25 ℃ -60 ℃)) = Approximately 23k Ω

I MOD = Is the modulation current you want to set wrong?

I don't understand the formula on page 18 (10) of the data sheet.

What is this formula for?

About ratio re: Is it Polarization Extinction Ratio?

Where did the value 6.3 come from?

(2) R MODTC

When used in open without implementation What kind of movement will it be?

(3) R BIAS MAX

To set the bias current to 6mA

From R BIASMAX [Ω] = 343V / I BIASMAX [A]

= 343V / 6mA = about 57k Ω

(4) R APCSET

From R APCSET [Ω] = 4.69V / PAVG [W] × ρMON [A / W]

= 4.69V / 0.5mW × 0.05 = Approximately 188k Ω

I don't understand much about the above, so I have no self-confidence.

Please give us some design advice.

  • +1
    TI__Guru* 93670 points

    Greetings,

    1). R MODSET:
    Equation 13 and 14 show how to set the modulation current with 4000 ppm temperature coefficient. In your example, you are using
    I MOD of 10mA and a value of 23K ohm for R MODSET is correct.

    2). Equation 10: Modulation current depends on the average optical power. extinction ratio(re), and the laser efficinecy.
    Given these parameters, you can calculate the required modulation current. Then equation 13 can be used to caculate R MODSET.
    Please note re is the extinction ratio - difference between power of logic "1" versus logic "0'.

    3). X6.3: This value is driven from re:
    8=10log(P1/P0) -> P1/P0 = 6.3

    4). R MODTC: In open-loop mode, the laser diode current is set by I BIASMAX and I MODSET. In this mode it is assumed that R MODTC is 630 ppm/c.

    5). R APCSET: In closed loop operation, this maintains I BIASMAX before a fault occurs. Please note equations 2 is used to calculate
    the bias compensation current. Note Monitor photo diode current = I BIASMAX - I PD.

    Regards ,,Nasser

  • +1 in reply to Nasser Mohammadi
    Prodigy 180 points

    Nasser san

    Thank you for your reply.

    >
    1). R MODSET:
    Equation 13 and 14 show how to set the modulation current with 4000 ppm temperature coefficient. In your example, you are using I MOD of 10mA and a value of 23K ohm for R MODSET is correct.

    ⇒ Where did the temperature coefficient of 4000ppm come from?

    > 2). Equation 10: Modulation current depends on the average optical power. Extinction ratio (re), and the laser efficinecy.
    Given these parameters, you can calculate the required modulation current. Then equation 13 can be used to caculate R MODSET.

    ⇒I understand.
    Modulation current is "extinction ratio (re), and the laser efficinecy"
    It means that it is calculated by substituting it into Equation 10.

    > Please note re is the extinction ratio --difference between power of logic "1" versus logic "0'.
    3). X6.3: This value is driven from re:
    8 = 10log (P1 / P0)-> P1 / P0 = 6.3

    ⇒I don't really understand here.
    I think re: 6.3 is a value obtained from 8 dB,
    Where did 8 dB come from?

    > 4). R MODTC: In open-loop mode, the laser diode current is set by I BIASMAX and I MODSET.

    ⇒ Using without implementing R MOD TC is called "open-loop mode"
    Do you express it?

    > In this mode it is assumed that R MODTC is 630 ppm / c.

    ⇒ What does this mean?

    > 5). R APCSET: In closed loop operation, this maintains I BIASMAX before a fault occurs.

    ⇒ Implementing and using R APC SET is called "closed loop operation".
    Do you express it?
    If this register is set, PAVG will always be kept constant.
    Is it correct?

    > Please note equations 2 is used to calculate the bias compensation current. Note Monitor photo diode current = I BIASMAX --I PD.

    ⇒ What does this mean?

    Regards ,,takahashi

  • +1 in reply to yoshihito takahashi
    TI__Guru* 93670 points

    Greetings,

    1). 4000ppm is part of the the laser diode's specification.

    3). 8dB is the desired - for example - extinction ratio. You can select other values.

    .3). In open loop mode, R MODTC is 630ppm/C.

    4). APC or Automatic Power Control maintains constant average power over temperature and laser diode temperature coefficients.

    Regards,Nasser

  • +1 in reply to Nasser Mohammadi
    Prodigy 180 points

    Nasser san

    Thank you for your reply.

    > 1). 4000ppm is part of the laser diode's specification.

    ⇒ About the temperature coefficient
    What parameters do you represent for temperature?

    ex.
    Temperature --Threshold current, Ith (mA)
    Temperature --Operating current, Iop (mA)
    Temperature --Slope efficiency, hs (mW / mA)
    Temperature --Operating voltage, Vop (V)
    Temperature --Monitor current, Is (mA)

    > 3). 8dB is the desired --for example --extinction ratio. You can select other values.

    ⇒I understand.

    > .3). In open loop mode, R MODTC is 630ppm / C.

    ⇒The temperature coefficient set by RMODTC and
    What is the difference in the temperature coefficient assigned by RMODSET? It was
    Data sheet page 19 (14)

    > 4). APC or Automatic Power Control maintains constant average power over temperature and laser diode temperature coefficients.

    ⇒I understand.

    Regards ,takahashi

  • +1 in reply to yoshihito takahashi
    TI__Guru* 93670 points

    Hi Takahashi-San,

    1). As temperature rises laser diode threshold  and efficiency is affected. During laser diode characterization, laser diode manufacturer obtain these data. I was able to find some paper online that you can refer too. There are some other app notes in this paper that could be helpful.

    https://www.newport.com/medias/sys_master/images/images/he9/hd7/8797049520158/AN05-Laser-Diode-Characteristics-Overview.pdf

    3).  In equation 14 of page 19 we should use a value of 630ppm/C if you operate the device in open-loop mode.

    Regards,,Nasser