光模块基础与光纤通信技术解析

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"光模块基础" 光模块是光通信系统中的核心组件，它负责光电转换和电光转换，使得数据能在光纤中传输。本技术培训手册详细阐述了光模块的基本原理和结构，适合对电子学有一定了解但对光纤技术不熟悉的读者。手册共分为六个部分，涵盖了光纤技术、网络技术、局域网标准、光纤类型与光传输、如何使用目录中的光纤部分以及光纤相关术语的词汇表。 1. 光纤技术：这部分介绍光纤通信的基础，包括光信号的传播方式、光纤的构造（如多模光纤和单模光纤的区别）、损耗和色散等关键概念。光纤的低损耗和大带宽特性使其成为长距离、高速率通信的理想选择。 2. 网络技术：这部分探讨了光纤在不同网络环境中的应用，如广域网、城域网和局域网。网络技术的发展推动了光模块的多样化，例如以太网光模块、FC光模块等，它们满足了不同速度和接口的需求。 3. 局域网标准：介绍了局域网（LAN）的相关标准，如IEEE 802.3系列，这些标准规定了光模块的数据速率、接口规范以及连接器类型，确保了设备间的兼容性。 4. 光纤类型与光传输：深入解析了不同类型光纤（如多模和单模）的特性和适用场景。同时，讨论了光信号在光纤中的传输方式，包括直接模态和格兰-汤普森效应等。 5. 如何使用目录中的光纤部分：这部分提供了一种指南，帮助用户理解和选择合适的光模块，包括考虑传输距离、接口类型、波长等因素，以适应特定的应用需求。 6. 词汇表：提供了丰富的光纤及相关技术术语，有助于读者更好地理解和学习光纤通信领域的专业概念。光模块的工作原理通常涉及发射端的激光二极管或光源将电信号转化为光信号，然后通过光纤传输；在接收端，光电探测器将接收到的光信号转化为电信号，恢复成原始数据。光模块的性能指标包括传输速率、工作距离、接口类型、光功率、灵敏度等，这些参数决定了光模块在实际网络环境中的表现。这份光模块基础的技术培训手册为读者提供了全面的光纤通信知识，不仅涵盖了理论基础，还指导了实践应用，是学习和理解光通信系统不可或缺的参考资料。

Each type of fiber has a different attenuation ver-

sus wavelength curve, none of which is linear or

exponential, but rather a complicated series of

peaks and valleys.

The attenuation of the light through a glass optical

waveguide has been decreased over the last 25

years to less than 0.02 db/km and scientists are

trying to decrease the attenuation even more.

The following graph depicts the attenuation for a speci-

fic type of plastic optical fiber. As the graph shows, cer-

tain wavelengths are ideal for transmission because they

fall in the valleys of the curve. For this type of plastic

fiber, the best suited wavelengths are 570 nm and 650

nm. For most glass fibers there are valleys around 850

nm, 1300 nm, and 1550 nm. For other types of fiber,

such as HPCS (Hard Plastic Clad Silica) fiber, the atten-

uation curve is relatively flat over a wide spectral range.

500 550 600 650 700 750 800

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

Attenuation

(dB/km)

Wavelen

(

)

This feature makes these types of fibers useful for a va-

riety of applications.

Attenuation loss in a fiber can be calculated by taking

the logarithmic ratio of the output

power (P

) to the input power (P

Indirect Measurements

Cutback Method

The attenuation of a fiber can be measured by transmit-

ting a signal through it and measuring the power at the

opposite end. The fiber is then cut near the input end

without changing the launch conditions and the power is

measured again. The loss per unit length can be calcu-

lated from the difference between these two values.

Substitution Method

A short length of reference fiber is compared with the

attenuation of the fiber being tested.

Both the cutback and substitution methods have draw-

backs. The attenuation measured by the cutback method

varies according to the numerical aperture and the spec-

tral bandwidth of the source. In the substitution method,

the coupling losses of the fiber being tested and the ref-

erence fiber may be different, so the value derived may

not be correct.

Direct Measurement

A more reliable technique for measuring attenuation in a

fiber is optical time-domain reflectometry (OTDR). In

this method, a brief signal (a short burst of optical

power) is introduced into the fiber. When this signal

encounters imperfections or discontinuities, some of the

light is reflected by a beamsplitter (a glass plate at a 45

degree angle to the incident light or two prisms cement-

ed together with metal-dielectric film between their

faces) onto a detector, where it is amplified and dis-

played on an oscilloscope.

The ODTR display is a smooth curve for a continuous

length of fiber and an irregular curve if discontinuities or

splices exist. The advantage of OTDR is that it yields

more accurate results.

Bit Error Ratio

In an actual transmission system, various elec-

tronic noise sources, such as the shot (random)

noise of the first transistor in the PIN-photodiode

preamplifier or noise on the 5-volt power-supply

line of the host digital system, can change or cor-

rupt the stream of data bits. The number of these

corrupted bits divided by the total number of re-

ceived bits within an arbitrary time period is the bit

error ratio (BER). The lower the BER value, the

fewer the number of errors in the transmission.

The bit-error ratio is usually given by a number such as

-6

. This means that on average, one error occurs for

every million pulses sent. Typical error ratios for opti-

cal fiber systems range from 10

-9

to 10

-12

(local-area

networks require a BER as low as 10

-12

). The BER can

be measured by repeatedly transmitting and receiving a

suitable length of a pseudo-random bit sequence (PRBS)

data. The necessary optical power to achieve a given

signal-to-noise ratio, and therefore a certain BER, is

called the receiver’s sensitivity.

dB = 10 log

Receiver Threshold-to-Noise Ratio vs. BER

1E-3 1E-5 1E-7 1E-9 1E-11 1E-13 1E-15

Threshold-to-Noise Ratio - (Vp-p/Vrms)

Bit-Error Ratio - (BER)

At any fixed temperature the total value of the receiver’s

random noise plus the host system’s noise can be as-

sumed to be a constant. So, the most obvious way to

reduce the probability of error is to increase the ampli-

tude of the optical signal applied to the receiver.

Another technique for lowering the error rate is to im-

prove the receiver’s ability to reject electrical noise from

the system it is in. One method is to use small PIN di-

ode pre-amps in the receiver’s first stage that do not

function as efficient antennas. A second method, for

extremely noisy applications, is to use PIN diode pre-

amps in electrically conductive plast5ic or all-metal

packages.

The worst source of noise is usually the host system’s

power (usually +5 V) supply. The host system’s +5 volt

supply normally powers the fiber-optic receiver, the fi-

ber-optic transmitter and an entire system comprised of

relatively noisy digital circuits. Simple and inexpensive

power supply filters have been proven to work in a wide

range of system applications. Simple, third-order pi

power-supply filters are normally sufficient to protect

the fiber-optic receiver from very noisy host systems, but

in extremely noisy applications, additional power supply

filtering might be needed.

Receiver sensitivity is specified at a BER; however, in

practice, the BER is too low to be directly measured in a

useful time period. For this reason, the transceiver is

usually tested during an artificially shorter time at a

much higher BER. The resulting data is then fitted to a

curve and the optical input power at the required BER is

extrapolated.

Note: The receiver’s signal-to-noise ratio determines

the probability of error; small increases in this ra-

tio will sharply reduce the probability of error.

Optical budgeting is a design technique that takes

system- and component-level parameters into ac-

count.

System-level parameters include:

- transmitter output power (P

dBm avg.),

- receiver sensitivity--the minimum optical power that

the receiver requires to operate at the specified data

rate and bit error ratio (P

dBm avg.),

- fiber losses (caused by attenuation, bandwidth and

insertion losses from connectors and are dependent

on the type of fiber) and

. - dynamic range.

Component-level parameters include:

- data rate,

- bit-error ratio (BER), and

- data pattern.

These two sets of parameters allow a designer to de-

termine the distance and power budgeting for the fi-

ber link.

Optical Power Budget

Power budget is the difference between the mini-

mum output power of the transmitter and the mini-

mum sensitivity of the receiver. If optical power

losses are greater than the worst-case power

budget, the BER increases and the reliability of the

link decreases.

Optical power expressed in µW or dBm, and optical

attenuation is expressed in dB/m. the total loss budget is

derived from the individual losses contributed by at-

tenuation and other possible losses in the link.

The Optical Power Margin is

M =

(φ

T -

) - (α

l + α

Optical Power Margin =

Optical Power Ratio - System Losses.

Note: For the fiber-optic cable to transmit and receive,

the flux margin must be positive.

Each of these losses is expressed in decibels (dB) as

Loss = 10 log

Pout

where Pin and Pout are the optical powers into and out

of the loss element.

Note: dBm is a unit used to describe optical power with

respect to one milliwatt. For example, if P

xµW), then

P = 10 Lo

dBm.

1000 (

The link loss budget considers the total optical power

loss allowed between the light source and the photode-

tector, and allocates this loss to cable attenuation, con-

nector loss, splice loss, and system margin.

The optical power budget is P

- P

, where P

is the

power launched by the transmitter and P

is the power

required by the receiver for proper operation. These

values are usually expressed in dBm, so the difference is

expressed in dB. If the losses between transmitter and

receiver exceed the power budget, then the power at the

receiver will not be adequate to assure proper operation.

In the following example, P

= -12 dBm and P

= -24

dBm. Substituting these values into the equation:

- P

= -12 - (-24) = -12 + 24 = +12 dB,

and so, the receiver has sufficient power to operate.

Eye Diagrams

An oscilloscope display of a long stream of ran-

dom bits (light pulses) is used to determine trans-

mitter and receiver performance. Due to distortion

of the waveform (by a variety of factors), the dis-

played light pulses do not align, but instead over-

lay one another and appear blurred, as shown be-

low.

The important point is that the center of the pattern

formed by these pulses must be open: the wider

the opening, the less the distortion and jitter.

Much information about system performance can be

gathered from the eye pattern:

- The width of the eye defines the time interval during

which the received signal can be sampled without er-

ror from inter-symbol interference.

- Amplitude distortion in the data signal reduces the

height of the eye opening. The smaller this dimen-

sion, the more difficult it is to detect the signal with-

out error.

- The height of the eye opening at the specified sam-

pling time shows the noise margin or immunity to

noise. Noise margin is derived from the following

equation:

Noise margin (percent) = X 100 percent

where V1 is equal to peak signal voltage and V2 is

equal to the ideal signal voltage.

Maximum

signal voltage

(V2)

Best sampling time

Noise margin (V1)

Time interval during

which signal can

be sampled

Threshold

Distortion at

sampling times

Distortion at

zero crossing

(

∆

∆∆

∆

Slope increases

sensitivity to

timing errors

- Timing jitter is caused by noise in the receiver and

pulse dispersion in the fiber. If the signal is meas-

ured at its midpoint, then this time-width indicates

the amount of jitter.

- The lower the slope of the signal’s transition time,

the more that the eye closes and the greater the in-

crease in timing errors. As the slope approaches the

horizontal, the possibility of error increases.

- If a truly random data stream passes through a linear

system, all eye openings will be identical and sym-

metrical. Any non-linearities will produce asymme-

try in the eye pattern.

Parameters such as rise- and fall-times, extinction ratio,

overshoot, undershoot, and pulse-width distortion can be

observed.

When an oscilloscope is triggered by the clock, the re-

sult is the classical “eye” pattern, as shown on the pre-

ceding page. Data are recoverable if the “eye” is “open”

above and below the threshold for a period of time

greater than the “set-up” and “hold” time needed for

subsequent decision devices or circuits.

Most serial communications standards now define a

minimum condition on eye-pattern opening (template)

for the serial data stream. The area from which the

waveform is excluded is the mask.

The mask limits that specify the eye-pattern opening are

an improvement over the older technique of specifying

pulse parameters individually. It is compact, conveying

important information without placing undue restrictions

on the data pulse. Limiting rise and fall times, pulse-

width distortion, overshoot, damping, undershoot, and so

forth, is unwieldy and insufficient by comparison.

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