Write-once media

Optical Data Storage on Write-once Media 607... 

CD-R disks are recordable disks that enable data to be written either once only, or in sessions on a multisession disk, allowing data to be updated and/or added to until the disk is full. However, the data on a CD-R disk cannot be erased or rewritten, hence CD-R disks are Write Once media, and are ideally suited to long-term data storage. 

Table 7.1. Performance Criteria for Write-Once Media...

To read out the recorded information with an adequate carrier-to-noise ratio (CNR), the medium must exhibit threshold recording behavior. The power level of the read beam can then be set at some value below threshold to guarantee nondestructive playback. For a variety of write-once media, the read power appears to be generally 3-5 times lower than the recording power. 

Write-once media, in most cases based on burning pits in an organic or inorganic layer which melts, evaporates or decomposes under the Influence of the laser beam. Another possibility is inducing a local phase transition, e.g. from polycrystalline to amorphous. Read-out is done at a lower intensity. 

FIGURE 14.21 Estimated lifetime of an IBM WORM disk. Lognormal plot of lifetimes for shelf and archival use. (Source Wong, J.S. et aL, 1993. Life expectancy of IBM write-once media. IBM White Paper, IBM Publications.)... 

Compact disk erasable/compact disk recordable (CD-E/CD-R) These define the writable versions of the compact disk. CD-E is a rewritable media and CD-R is a write-once media. 

The problem with the application of these changes in optical properties for optical data storage is that the significant drop in reflectivity upon the first UV irradiation is related to an irreversible structural change. So the film can be used a as write-once medium only. Furthermore, the sensitivity of the system is low, so writing the information takes a rather long time. The changes in the optical properties in subsequent irradiation cycles are reversible, but they are connected with the unstable cis-isomer, which renders the inscribed information volatile. 

A viable real-time recording disk medium was developed in the mid 1980s, which became known as WORM (Write Once Read Many). However, there was no common standard for the medium, hence media produced by different manufacturers were incompatible, each requiring its own unique hardware. It is therefore not surprising that growth in this area was restricted and prices remained high.196 This is in direct contrast to the industry-wide standards which helped lead to the successful introduction of CD technology. 

Two naphthalocyanine derivatives (Scheme 9) were mixed together to improve read-out stability in a write-once optical storage system.220 The Ge derivative has a bulky structure with large side chains on the central atom, whilst the Cu derivative has a flat structure. The stability of the medium under 1 mW laser irradiation for read-out was improved 100-fold compared to that of a single-component system.220... 

The mechanism of WORM recording and indeed of all currently available optical disc media, whether erasible or write-once, depends upon the heat generated by a focused laser beam impinging on an absorbing medium. The energy, normally near-IR radiation, is transmitted to the absorbing material in a time much shorter than the time it takes for the heat generated to dissipate by thermal conduction. 

Figure 9. Schematic of write-once bubble-forming medium.

As a final example, consider line D of Table 9.1. We represent this problem as a body of density p, and heat capacity cp and whose surface is in contact with another medium of temperature Ts. Assume the initial body temperature is the same as the temperature of the other medium at I m = Ts. From the fundamental equation we can write, pcpLdTb/dt = X(TS — Tb)/L, where L is the characteristic conductive length, and X is the thermal conductivity. We now scale this problem over the entire time of the thermal transient. Once the entire time of the transient passes fe - t ), the body will have reached the new temperature of 7, 2. For the overall transient, the temperature rate of change is (Tb2 - Tb )/Gi -t ). and the average driving potential for the thermal conduction will be TS2 - T, )/2 = ( 7),2 - Tj, )/2. We now define the first-order relationship between the parameter as... 

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